Origins, the 8-inch disk
In 1967 IBM gave their San Jose, California storage development center a new task: develop a simple and inexpensive system for loading microcode into their System/370 mainframes. The 370s were the first IBM machines to use semiconductor memory, and whenever the power was turned off the microcode had to be reloaded ('magnetic core' memory, used in the 370s' predecessors, the System/360 line, did not lose its contents when powered down). Normally this task would be left to various tape drives which almost all 370 systems included, but tapes were large and slow. IBM wanted something faster and more purpose-built that could also be used to send out updates to customers for $5.
Alan Shugart, the overall IBM Product Manager, assigned this to David Noble who tried a number of solutions to see if he could develop a new-style tape or other media for the purpose, but eventually gave up. Noble's team then invented a read-only, 8-inch (20 cm) floppy they called the "memory disk", holding 80 kilobytes. The original versions were simply the disk itself, but dirt became a serious problem and they enclosed it in a plastic envelope lined with fabric that would pick up the dirt. The new device, developed under the code name Minnow and announced as the 23FD, became a standard part of 370 systems starting in 1969. It was also used as a program load device for other IBM products such as the 2835 Storage Control Unit.
Alan Shugart left IBM, moved to Memorex where his team in 1972 shipped the Memorex 650, the first read-write floppy disk drive.
In 1973 IBM released a new version of the floppy, this time on the 3740 Data Entry System. The new system used a different recording format that stored up to 250 kB on the same disks, and was read-write. These drives became common, and soon were being used to move smaller amounts of data around, almost completely replacing magnetic tapes.
The IBM standard soft-sectored disk format was designed to hold just as much data as one box of punch cards. The disk was divided into 77 tracks of 26 sectors, each holding 128 bytes. Note that 77x26 = 2002 sectors, whereas a box of punch cards held 2000 cards.
When the first microcomputers were being developed in the 1970s, the 8-inch floppy found a place on them as one of the few "high speed, mass storage" devices that were even remotely affordable to the target market (individuals and small businesses). The first microcomputer operating system, CP/M, originally shipped on 8-inch disks. However, the drives were still expensive, typically costing more than the computer they were attached to in early days, so most machines of the era used cassette tape instead.
This began to change with the acceptance of the first standard for the floppy disk, ECMA-59, authored by Jim O'Reilly of Burroughs, Helmuth Hack of BASF and others. O'Reilly set a record for maneuvering this document through ECMA's approval process, with the standard sub-committee being formed in one meeting of ECMA and approval of a draft standard in the next meeting three months later. This standard later formed the basis for the ANSI standard too. Standardization brought together a variety of competitors to make media to a single interchangeable standard, and allowed rapid quality and cost improvement.
Shugart moved on in 1973 to found Shugart Associates. They started working on improvements to the existing 8-inch format, eventually creating a new 800 kB system. However, profits were hard to find, and in 1974 he was forced out of his own company.
Burroughs Corporation, meanwhile, was developing a high-performance dual-sided 8-inch drive at their Glenrothes, Scotland factory. With a capacity of 1 MB (MiB), this unit exceeded IBM's drive capacity by 4 times, and was able to provide enough space to run all the software and store data on the new Burrough's B80 data entry system, which incidentally had the first VLSI disk controller in the industry. The dual-sided 1MB floppy entered production in 1975, but was plagued by an industry problem, poor media quality. There were few tools available to test media for 'bit-shift' on the inner tracks, which made for high error rates, and the result was a substantial investment by Burroughs in a media tester designed by Dr Nigel Mackintosh (who later made important contributions to the science of disk drive testing using Phase Margin Analysis) that they then gave to media makers as a quality control tool, leading to a vast improvement in yields.
The 5¼-inch minifloppy (5.25-inch floppy)
In 1975, Burroughs' plant in Glenrothes developed a prototype 5¼-inch drive, stimulated both by the need to overcome the larger 8-inch floppy's asymmetric expansion properties with changing humidity, and to reflect the knowledge that IBM's audio recording products division was demonstrating a dictation machine using 5¼-inch disks. In one of the industry's historic gaffes, Burroughs corporate management decided it would be "too inexpensive" to make enough money, and shelved the program.
In 1976 two of Shugart Associates's employees, Jim Adkisson and Don Massaro, were approached by An Wang of Wang Laboratories, who felt that the 8-inch format was simply too large for the desktop word processing machines he was developing at the time. After meeting in a bar in Boston, Adkisson asked Wang what size he thought the disks should be, and Wang pointed to a napkin and said "about that size". Adkisson took the napkin back to California, found it to be 5¼-inches (13 cm) wide, and developed a new drive of this size storing 98.5 KB later increased to 110 KB by adding 5 tracks. This is believed to be the first standard computer medium that was not promulgated by IBM.
The 5¼-inch drive was considerably less expensive than 8-inch drives from IBM, and soon started appearing on CP/M machines. At one point Shugart was producing 4,000 drives a day. By 1978 there were more than 10 manufacturers producing 5¼-inch floppy drives, in competing physical disk formats: hard-sectored (90 KB) and soft-sectored (110 KB). The 5¼-inch formats quickly displaced the 8-inch from most applications, and the 5¼-inch hard-sectored disk format eventually disappeared.
These early drives read only one side of the disk, leading to the popular budget approach of cutting a second write-enable slot and index hole into the carrier envelope and flipping it over (thus, the "flippy disk") to use the other side for additional storage. This was considered risky by some, the reasoning being that when flipped the disk would spin in the opposite direction inside its cover, so some of the dirt that had been collected by the fabric lining in the previous rotations would be picked up by the disk and dragged past the read/write head. In reality, since some single-head floppy drives had their read/write heads on the bottom and some had them on the top, disk manufacturers routinely certified both sides of disks for use, thus the method was perfectly safe.
For most of the 1970s and 1980s the floppy drive was the primary storage device for microcomputers. Since these micros had no hard drive, the OS was usually booted from one floppy disk, which was then removed and replaced by another one containing the application. Some machines using two disk drives (or one dual drive) allowed the user to leave the OS disk in place and simply change the application disks as needed. In the early 1980s, "quad density" 96 track-per-inch drives appeared, increasing the capacity to 720 KB. Another oddball format was used by Digital Equipment Corporation's Rainbow-100, DECmate-II and Pro-350. It held 400 KB on a single side by using 96 tracks-per-inch and cramming 10 sectors per track.
Despite the available capacity of the disks, support on the most popular operating system of the early 80's - PC-DOS and MS-DOS - lagged slightly behind. In fact, the original IBM PC did not include a floppy drive at all as standard equipment - you could either buy the optional 5¼-inch floppy drive or rely upon the cassette port. With version 1.0 of DOS (1981) only single sided 160 KB floppies were supported. Version 1.1 the next year saw support expand to double-sided, 320 KB disks. Finally in 1983 DOS 2.0 supported 9 sectors per track rather than 8, providing 180 KB on a (formatted) single-sided disk and 360 KB on a double-sided. Along with this change came support for different directories on the disk (now commonly called folders), which came in handy when organizing the greater number of files possible in this increased space.
In 1984, along with the IBM PC/AT, the high density disk appeared, which used 96 tracks per inch combined with a higher density magnetic media to provide 1,200 KB of storage (formerly referred to as 1.2 megabytes). Since the usual (very expensive) hard disk held 10-20 megabytes at the time, this was considered quite spacious.
By the end of the 1980s, the 5¼-inch disks had been superseded by the 3½-inch disks. Though 5¼-inch drives were still available, as were disks, they faded in popularity as the 1990s began. The main community of users was primarily those who still owned '80s legacy machines (PCs running MS-DOS or home computers) that had no 3½-inch drive; the advent of Windows 95 (not even sold in stores in a 5¼-inch version; a coupon had to be obtained and mailed in) and subsequent phaseout of standalone MS-DOS with version 6.22 forced many of them to upgrade their hardware. On most new computers the 5¼-inch drives were optional equipment. By the mid-1990s the drives had virtually disappeared as the 3½-inch disk became the preeminent floppy disk.
The "Twiggy" disk
During the development of the Apple Lisa, Apple developed a disk format codenamed Twiggy. While basically similar to a standard 5.25in disk, the Twiggy disk had an additional set of write windows on the top of the disk with the label running down the side. The drive was also present in prototypes of the original Apple Macintosh computer, but was removed in both the Mac and later versions of the Lisa in favor of the 3.5in floppy disk from Sony. The drives were notoriously unreliable and Apple was criticized for needlessly diverging from industry standards.
New formats, no standard
Throughout the early 1980s the limitations of the 5¼-inch format were starting to become clear. Originally designed to be a smaller and more practical 8-inch, the 5¼-inch system was itself too large, and as the quality of the recording media grew, the same amount of data could be placed on a smaller surface. Another problem was that the 5¼-inch disks were simply copies of the 8-inch physical format, which had never really been engineered for ease of use. The thin folded-plastic shell allowed the disk to be easily damaged through bending, and allowed dirt to get onto the disk surface through the opening.
A number of solutions were developed, with drives at 2-inch, 2½-inch, 3-inch and 3½-inch (50, 60, 75 and 90 mm) all being offered by various companies. They all shared a number of advantages over the older format, including a small form factor and a rigid case with a slideable write protect catch. The almost-universal use of the 5¼-inch format made it very difficult for any of these new formats to gain any significant market share.
Standard 3-inch and 3½-inch disks used the same spin speed and basic hardware interface as standard 5¼-inch drives, allowing them to be used with existing controllers and formats, although new formats were later developed that relied on the higher quality hardware in the new drive types (the IBM PC in particular never officially shared a format between the two drive types, though it was possible to misidentify the drive to the OS if desired).
The 3-inch compact floppy disk
The original concept of the 3-inch hard case floppy disk was developed in 1973 by Marcell Jánosi, a Hungarian inventor of Budapest Radiotechnic Company (Budapesti Rádiótechnikai Gyár - BRG). The system was the BRG MCD-1, which was patented but later the patent was not extended, therefore the protection was lost and Amdek released the AmDisk-3 Micro-Floppy-disk cartridge system in December 1982. It was designed for use with the Apple II Disk II interface card, but has also been successfully connected to other computers.
The drive itself was manufactured by Hitachi, Matsushita and Maxell. Only Teac outside this "network" is known to have produced drives. Similarly, only three manufacturers of media (Maxell, Matsushita and Tatung) are known (sometimes also branded Yamaha, Amsoft, Panasonic, Tandy, Godexco and Dixons), but "no-name" disks with questionable quality have been seen in circulation.
Amstrad included a 3-inch single-sided, double-density (180 KB) drive in their CPC and some models of PCW. The PCW-8512 included a double-sided, quad density (720 KB) as the second drive and later models, such as the PCW-9512 used quad density even for the first drive. The single-sided double density (180 KB) drive was "inherited" by the ZX Spectrum +3 computer after Amstrad bought the rights from Sinclair.
While all 3-inch media were double-sided in nature, single-sided drive owners were able to flip the disk over to use the other side. The sides were termed "A" and "B" and were completely independent, but single-sided drive units could only access the upper side at one time.
The disk format itself had no more capacity than the more popular (and cheaper) 5¼-inch floppies. Each side of a double-density disk held 180 KB for a total of 360 KB per disk, and 720 KB for quad-density disks. Unlike 5¼-inch or 3½-inch disks, the 3-inch disks were designed to be reversible and sported two independent write-protect switches. It was also more reliable thanks to its hard casing.
3-inch drives were also used on a number of exotic and obscure CP/M systems such as the Tatung Einstein and occasionally on MSX systems in some regions. Other computers to have used this format are the more unknown Gavilan Mobile Computer and Matsushita's National Mybrain 3000. The Yamaha MDR-1 also used 3-inch drives.
The main problems with this format was the high price, due to the quite elaborate and complex case mechanisms. However, the tip on the weight was when Sony in 1984 convinced Apple Computer to use the 3½-inch drives in the Macintosh 128K model, effectively making the 3½-inch drive a de-facto standard.
Mitsumi's "Quick Disk" 3-inch floppies
Another 3-inch format was Mitsumi's Quick Disk format. The Quick Disk format is referred to in various size references: 2.8-inch, 3-inchx3-inch and 3-inchx4-inch. Confusing when trying to categorize the disk but perhaps not when understood that Mitsumi offered this as OEM equipment, expecting their VAR customers to customize the packaging for their own particular use. Nintendo packaged the 2.8-inch magnetic media in a 3-inchx4-inch housing, while others packaged the same media in a 3?x3? housing. This explains the different numbering labels, while here we generically call the Mitsumi Quick Disk a 3-inch format.
The Quick Disk's most successful use was in Nintendo's Famicom Disk System. The FDS package of Mitsumi's Quick Disk used a 3-inchx4-inch plastic housing called the "Disk System Card". Most FDS disks did not have cover protection to prevent media contamination, but a later special series of five games did include a protective shutter.
Mitsumi's "3-inch" Quick Disk media was also used in a 3-inchx3-inch housing for many Smith Corona word processors. The Smith Corona disks are confusingly labeled "DataDisk 2.8 inch", presumably referring to the size of the media inside the hard plastic case.
The Quick Disk was also used in several MIDI keyboards and MIDI samplers of the mid 1980s. A non-inclusive list includes: the Roland S-10 and MKS100 samplers, the Korg sqd1, the Korg SQD8 MIDI sequencer, Akai's 1985 model MD280 drive for the S-612 MIDI Sampler, Akai's X7000 / S700 (rack version) and X3700, the Roland S-220, and the Yamaha MDF1 MIDI disk drive (intended for their DX7/21/100/TX7 synthesizers, RX11/21/21L drum machines, and QX1, QX21 and QX5 MIDI sequencers).
As the cost in the 1980s to add 5.25-inch drives was still quite high, the Mitsumi Quick Disk was competing as a lower cost alternative packaged in several now obscure 8-bit computer systems. Another non-inclusive list of Quick Disk versions: QDM-01, QDD (Quick Disk Drive) on french Thomson micro-computers, in the Casio QD-7 drive, in a peripheral for the Sharp MZ-700 & MZ-800 system, in the DPQ-280 Quickdisk for the Daewoo/Dynadata MSX1 DPC-200, in a Dragon machine, in the Crescent Quick Disk 128, 128i and 256 peripherals for the ZX Spectrum, and in the Triton Quick Disk peripherial also for the ZX Spectrum and ZX Spectrum.
The World of Spectrum FAQ reveals that the drives did come in different sizes: 128 to 256 kB in Cresent's incarnation, and in the Triton system, with a density of 4410 bpi, data transmission rate of 101.6 kb/s, a 2.8-inch double sided disk type and a capacity of up to 20 sectors per side at 2.5 kB per sector, up to 100 kB per disk. Quick Disk as used in the Famicom Disk System holds 64 kB of data per side, requiring a manual turn-over to access the second side.
It is significant to note that the Quick Disk utilizes "a continuous linear tracking of the head and thus creates a single spiral track along the disk similar to a record groove." This has led some to compare it more to a "tape-stream" unit than typically what is thought of as a random-access disk drive.
The 3½-inch microfloppy diskette
Sony introduced their own small-format 90.0 x 94.0 mm disk, similar to the others but somewhat simpler in construction than the AmDisk. The first computer to use this format was the HP-150 of 1983, and Sony also used them fairly widely on their line of MSX computers. Other than this the format suffered from a similar fate as the other new formats; the 5¼-inch format simply had too much market share. Things changed dramatically in 1984 when Apple Computer selected the format for their new Macintosh computers. By 1988 the 3½-inch was outselling the 5¼-inch.
Note that the term "3½-inch" or "3.5 inch" disk was primarily targeted at the non-metric US market and was rounded from the actual metric size of 90 mm used internationally.
The 3½-inch disks had, by way of their rigid case's slide-in-place metal cover, the significant advantage of being much better protected against unintended physical contact with the disk surface than 5¼-inch disks when the disk was handled outside the disk drive. When the disk was inserted, a part inside the drive moved the metal cover aside, giving the drive's read/write heads the necessary access to the magnetic recording surfaces. Adding the slide mechanism resulted in a slight departure from the previous square outline. The irregular, rectangular shape had the additional merit that it made it impossible to insert the disk sideways by mistake as had indeed been possible with earlier formats.
The shutter mechanism was not without its problems, however. On old or roughly treated disks the shutter could bend away from the disk. This made it vulnerable to being ripped off completely (which does not damage the disk itself but does leave it much more vulnerable to dust), or worse, catching inside a drive and possibly either getting stuck inside or damaging the drive. On disks with the cover bending away the best option is to rip the cover off (to make sure it does not catch in the drive) and then immediately copy the data off it. Most modern floppies have a springy plastic cover that does not tend to bend away from the disk.
Like the 5¼-inch, the 3½-inch disk underwent an evolution of its own. When Apple introduced the Macintosh in 1984, it used single-sided 3½-inch disk drives with an advertised capacity of 400 kB. The encoding technique used by these drives was known as GCR, or Group Code Recording. Somewhat later, PC-compatible machines began using single-sided 3½-inch disks with an advertised capacity of 360 kB (the same as a single-sided 5¼-inch disk), and a different, incompatible recording format called MFM (Modified Frequency Modulation). GCR and MFM drives (and their formatted disks) were incompatible, although the physical disks were the same. In 1986, Apple introduced double-sided, 800 kB disks, still using GCR, and around the same time, 720 kB double-sided double-density MFM disks began to appear on PC-compatibles.
A newer and better "high-density" format, displayed as "HD" on the disks themselves and storing 1440 kB of data, was introduced in 1987. These HD disks had an extra hole in the case on the opposite side of the write-protect notch. IBM used this format on their PS/2 series introduced in 1987. Apple started using "HD" in 1988, on the Macintosh IIx, and the HD floppy drive soon became universal on virtually all Macintosh and PC hardware. Apple's HD drive was capable of reading and writing both GCR and MFM formatted disks, and thus made it relatively easy to exchange files with PC users. Apple marketed this drive as the "SuperDrive." Interestingly, Apple began using the SuperDrive brand name again around 2003 to denote their all-formats CD/DVD reader/writer.
Another advance in the oxide coatings allowed for a new "extended-density" ("ED") format at 2880 kB introduced on the second generation NeXT Computers in 1991, and on IBM PS/2 model 57 also in 1991, but by the time it was available it was already too small in capacity to be a useful advance over the HD format and never became widely used. The 3½-inch drives sold more than a decade later still use the same 1.44 MB HD format that was standardized in 1989, in ISO 9529-1,2.
Reported 3.5" DSHD FDD storage capacity
The unformatted capacity of 3½-inch double sided high density floppy disk is 2.0 megabytes; in its most common format it has a capacity of 1,474,560 bytes or 1.47 MB (simply dividing by 1,000,000). In a binary prefix numbering system this is 1.41 MebiByte.
However, neither of these numbers is generally used. The number most frequently printed on such floppies is 1.44 MB. This value was apparently reached by doubling (in the decimal system) the capacity of the prior generation 720 "KB" [actually, KiB] double sided double density floppy disk and dividing by 1,000, to arrive at 1.44 and mis-labeling such as "MB" [actually, thousands of KiB's]. A person expecting the 1.44 "MB" number to be stated in either the binary prefix or the decimal prefix would always miscalculate the number of floppies needed.
Through the early 1990s a number of attempts were made by various companies to introduce newer floppy-like formats based on the now-universal 3½-inch physical format. Most of these systems provided the ability to read and write standard DD and HD disks, while at the same time introducing a much higher-capacity format as well. There were a number of times where it was felt that the existing floppy was just about to be replaced by one of these newer devices, but a variety of problems ensured this never took place. None of these ever reached the point where it could be assumed that every current PC would have one, and they have now largely been replaced by CD and DVD burners and USB flash drives.
The main technological change was the addition of tracking information on the disk surface to allow the read/write heads to be positioned more accurately. Normal disks have no such information, so the drives use the tracks themselves with a feedback loop in order to center themselves. The newer systems generally used marks burned onto the surface of the disk to find the tracks, allowing the track width to be greatly reduced.
As early as 1988, Brier Technology introduced the Flextra BR 3020, which boasted 21.4 MB (marketing, true size was 21,040 kiB, 25 MiB unformatted). Later the same year it introduced the BR3225, which doubled the capacity. This model could also read standard 3½-inch disks.
Apparently it used 3½-inch standard disks which had servo information embedded on them for use with the Twin Tier Tracking technology.
In 1991, Insite Peripherals introduced the "Floptical", which used an infra-red LED to position the heads over marks in the disk surface. The original drive stored 21 MiB, while also reading and writing standard DD and HD floppies. In order to improve data transfer speeds and make the high-capacity drive usefully quick as well, the drives were attached to the system using a SCSI connector instead of the normal floppy controller. This made them appear to the operating system as a hard drive instead of a floppy, meaning that most PCs were unable to boot from them. This again adversely affected pickup rates.
Insite licenced their technology to a number of companies, who introduced compatible devices as well as even larger-capacity formats. Most popular of these, by far, was the LS-120, mentioned below.
In 1994, Iomega introduced the Zip drive. Not true to the 3½-inch form factor, hence not compatible with the standard 1.44 MB floppies (which may have actually been a good thing for the drives as it removed a big potential source of problems), it became the most popular of the "super floppies". It boasted 100 MB, later 250 MB, and then 750 MB of storage and came to market at just the right time, with Zip drives gaining in popularity for several years. It never reached the same market penetration as floppy drives, as only a few new computers were sold with Zip drives. Eventually the falling prices of CD-R and CD-RW media and flash drives, and notorious hardware failures (the so-called "click of death") reduced the popularity of the the Zip drive.
A major reason for the failure of the Zip Drives is also attributed to the higher pricing they carried. However hardware vendors such as Hewlett Packard, Dell and Compaq had promoted the same at a very high level. Zip drive media was primarily popular for the excellent storage density and drive speed they carried, but was always overshadowed by the price.
Announced in 1995, the "SuperDisk" drive, often seen with the brand names Matsushita (Panasonic) and Imation, had an initial capacity of 120 MB (120.375 MiB) using even higher density "LS-120" disks.
It was upgraded ("LS-240") to 240 MB (240.75 MiB). Not only could the drive read and write 1440 kB disks, but the last versions of the drives could write 32 MB onto a normal 1440 kB disk. Unfortunately, popular opinion held the Super Disk disks to be quite unreliable, though no more so than the Zip drives and SyQuest Technology offerings of the same period and there were also many reported problems moving standard floppies between LS-120 drives and normal floppy drives. This again, true or otherwise, crippled adoption.
Sony introduced their own floptical-like system in 1997 as the 150 MiB Sony HiFD. Although by this time the LS-120 had already garnered some market penetration, industry observers nevertheless confidently predicted the HiFD would be the real floppy-killer and finally replace floppies in all machines.
After only a short time on the market the product was pulled as it was discovered there were a number of performance and reliability problems that made the system essentially unusable. Sony then re-engineered the device for a quick re-release, but then extended the delay well into 1998 instead and increased the capacity to 200 MiB while they were at it. By this point the market was already saturated by the Zip disk so it never gained much market share.
Caleb Technology's UHD144
The UHD144 drive surfaced early in 1998 as the it drive, and provided 144 MB of storage while also being compatible with the standard 1.44 MB floppies. The drive was slower than its competitors but the media was cheaper, running about $8 at introduction and $5 soon after.
Current situation (2007)
The 8-inch, 5¼-inch and 3-inch formats can be considered almost completely obsolete. 3½-inch drives and disks are still widely available. As of 2006 3½-inch drives are still available on many desktop PC systems, although it is usually now an optional extra or has to be bought and installed separately. HP has recently dropped supplying floppy drives as standard on business desktops. The majority of ATX and Micro-ATX PC cases are still designed to accommodate at least one 3.5" drive that can be accessed from the front of the PC (although this can be used for other devices than just floppy drives). As of 2006, HD floppy disks are still quite commonly available in most computer and stationery shops, although selection is usually very limited.
Floppy disks still maintain a stronghold when it comes to emergency boots, BIOS updates and as maintenance program carriers, in general, as many BIOS and firmware update/restore programs are still designed to be executed from a bootable floppy disk, and the legacy support for alternate bootable media such as CD-ROMs and USB devices is still problematic in some configurations.
Floppy disks are still an essential part of setting up a new computer system from the ground up, as, for example, even comparatively recent operating systems like Windows XP rely on third party drivers shipped on floppy disks, a typical example being SATA support during installation.
Perhaps the longevity of the 3½-inch disk is also due in part to the music industry where other types of electronic equipment use this format as a storage medium. Musical equipment such as synthesizers, samplers, drum machines, sequencers, etc. continue to use the 3½-inch disk. Other storage options for musical equipment, such as CD-R, CD-RW, network connections, and USB storage devices have taken much longer to mature in this industry.
However, the advent of other portable storage options, such as USB storage devices and recordable or rewritable CDs, and the rise of multi-megapixel digital photography have encouraged the creation and use of files larger than most 3½-inch disks can hold. In addition, the increasing availability of broadband and wireless Internet connections is decreasing the utility of removable storage devices overall. The 3½-inch floppy is growing as obsolete as its larger cousin became a decade before. However, the 3½-inch floppy has been in continued use longer than the 5¼-inch floppy.
Some manufacturers have stopped offering 3½-inch drives on new computers as standard equipment. The Apple Macintosh, which popularized the format in 1984, stopped including 3½-inch drives in new models, beginning, in 1998, with the iMac. This made USB-connected floppy drives a popular accessory for the early iMacs, in part since the basic model iMac of the time only had a CD-ROM drive, giving users no easy access to removable media. In February 2003, Dell, Inc. announced that they would no longer include floppy drives on their Dell Dimension home computers as standard equipment, although they are available as a selectable option for around $20 and can be purchased as an aftermarket OEM addon anywhere between $5 and $25.
On 29 January 2007 the British computer retail chain PC World issued a statement saying that only 2% of the computers that they sold contained a built-in floppy disk drive and, once present stocks were exhausted, no more floppies would be sold.
In general, different physical sizes of floppy disks are incompatible by definition, and disks can be loaded only on the correct size of drive. There were some drives available with both 3½-inch and 5¼-inch slots that were popular in the transition period between the sizes.
However, there are many more subtle incompatibilities within each form factor. For example, all but the earliest models of Apple Macintosh computers that have built-in floppy drives included a disk controller that can read, write and format IBM PC-format 3½-inch diskettes. However, few IBM-compatible computers use floppy disk drives that can read or write disks in Apple's variable speed format.
Within the world of IBM-compatible computers, the three densities of 3½-inch floppy disks are partially compatible. Higher density drives are built to read, write and even format lower density media without problems, provided the correct media is used for the density selected. However, if by whatever means a diskette is formatted at the wrong density, the result is a substantial risk of data loss due to magnetic mismatch between oxide and the drive head's writing attempts. Still, a fresh diskette that has been manufactured for high density use can theoretically be formatted as double density, but only if no information has ever been written on the disk using high density mode (for example, HD diskettes that are pre-formatted at the factory are out of the question). The magnetic strength of a high density record is stronger and will "overrule" the weaker lower density, remaining on the diskette and causing problems. However, in practice there are people who use downformatted (ED to HD, HD to DD) or even overformatted (DD to HD) without apparent problems. Doing so always constitutes a data risk, so one should weigh up the benefits (e.g. increased space and/or interoperability) versus the risks (data loss, permanent disk damage).
The situation was even more complex with 5¼-inch diskettes. The head gap of an 80 track (1200 kB in the PC world) drive is shorter than that of a 40 track (360 kB in the PC world) drive, but will format, read and write 40 track diskettes with apparent success provided the controller supports double stepping (or the manufacturer fitted a switch to do double stepping in hardware). A blank 40 track disk formatted and written on an 80 track drive can be taken to a 40 track drive without problems, similarly a disk formatted on a 40 track drive can be used on an 80 track drive. But a disk written on a 40 track drive and updated on an 80 track drive becomes permanently unreadable on any 360 kB drive, owing to the incompatibility of the track widths (special, very slow programs could have been used to overcome this problem). There are several other 'bad' scenarios.
Prior to the problems with head and track size, there was a period when just trying to figure out which side of a "single sided" diskette was the right side was a problem. Both Radio Shack and Apple used 360 kB single sided 5¼-inch disks, and both sold disks labeled "single sided" were certified for use on only one side, even though they in fact were coated in magnetic material on both sides. The irony was that the disks would work on both Radio Shack and Apple machines, yet the Radio Shack TRS-80 Model I computers used one side and the Apple II machines used the other, regardless of whether there was software available which could make sense of the other format.
For quite a while in the 1980s, users could purchase a special tool called a "disk notcher" which would allow them to cut a second "write unprotect" notch in these diskettes and thus use them as "flippies" (either inserted as intended or upside down): both sides could now be written on and thereby the data storage capacity was doubled. Other users made do with a steady hand and a hole punch or scissors. For re-protecting a disk side, one would simply place a piece of opaque tape over the notch or hole in question. These "flippy disk procedures" were followed by owners of practically every home-computer single sided disk drives. Proper disk labels became quite important for such users. Flippies were eventually adopted by some manufacturers, with a few programs being sold in this media (they were also widely used for software distribution on systems that could be used with both 40 track and 80 track drives but lacked the software to read a 40 track disk in an 80 track drive).