A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles. They are read and written by a floppy disk drive (FDD).

Invented by IBM, floppy disks in 3.5-inch (89 mm), 5.25-inch (133 mm) and 8-inch (200 mm) forms were a ubiquitous form of data storage and exchange from the mid-1970s to the 2000s.

While floppy disk drives still have some limited uses, especially with legacy industrial computer equipment, they have been superseded by data storage methods with much greater capacity, such as USB flash drives, portable external hard disk drives, optical discs, memory cards, and computer networks.

Floppy disks are used for emergency boots in aging systems lacking support for other bootable media, and for BIOS updates since most BIOS and firmware programs can still be executed from bootable floppy disks. If BIOS updates fail or become corrupt, floppy drives can be used to perform a recovery. The music and theatre industries still use equipment requiring standard floppy disks (e.g. synthesizers, samplers, drum machines, sequencers, and lighting consoles). Industrial automation equipment such as programmable machinery and industrial robots may not have a USB interface; data and programs are then loaded from disks, damageable in industrial environments. This may not be replaced due to cost or requirement for continuous availability; existing software emulation and virtualization do not solve this problem because no operating system is present or a customized operating system is used that has no drivers for USB devices. Hardware floppy disk emulators can be made to interface floppy disk controllers to a USB port that for flash drives; several manufacturers make such emulators.

The 5¼-inch disk has a large circular hole in the center for the drive’s spindle and a small oval aperture in both sides of the plastic to allow the drive’s heads to read and write data; the magnetic medium can be spun by rotating it from the middle hole. A small notch on the right of the disk identifies that it is writable, detected by a mechanical switch or phototransistor above it; if it is not present, the disk is read-only. Punch devices were sold to convert read-only disks to writable ones and enable writing on the unused side of single sided disks; such modified disks became known as flippy disks. Tape may be used over the notch to protect writable disks from unwanted writing.

Another LED/photo-transistor pair located near the center of the disk detects the index hole once per rotation in the magnetic disk; it is used to detect the angular start of each track and whether or not the disk is rotating at the correct speed. Early 8-inch and 5¼-inch disks had physical holes for each sector and were termed hard sectored disks. Later soft sectored disks had only one index hole, and sector position was determined by the disk controller or low level software from patterns marking the start of a sector. Generally, the same drives were used to read and write both types of disks, with only the disks and disk controllers differing. Some operating systems utilizing soft sectors, such as Apple DOS, did not use the index hole; the drives designed for such systems often lacked the corresponding sensor; this was mainly a hardware cost saving measure.

Inside the disk are two layers of fabric, with the medium sandwiched in the middle. The fabric is designed to reduce friction between the medium and the outer casing, and catch particles of debris abraded off the disk to keep them from accumulating on the heads. The outer casing is usually a one-part sheet, double-folded with flaps glued or spot-welded together. The 8-inch disk had read-only logic that was the reverse of the 5¼-inch disk, with the slot on the side having to be taped over to allow writing.

The core of the 3½-inch disk is the same as the other two disks, but the front has only a label and a small aperture for reading and writing data, protected by the slider – a spring-loaded metal or plastic cover, pushed to the side on entry into the drive. Rather than having a hole in the center, it has a metal hub which mates to the spindle of the drive.

Typical 3½-inch disk magnetic coating materials are:

* DD: 2 µm magnetic iron oxide (Coercivity approx. 300 OE Oersted)
* HD: 1.2 µm cobalt doped iron oxide (Coercivity approx. 600 OE)
* ED: 3 µm Barium ferrite (Coercivity approx. 750 OE)

Two holes at the bottom left and right indicate whether the disk is write-protected and whether it is high-density; these holes are spaced as far apart as the holes in punched A4 paper, allowing write-protected high-density floppies to be clipped into standard ring binders. A notch at top right ensures that the disk is in the correct orientation and an arrow at top left indicating direction of insertion. The drive usually has a button that when pressed ejects the disk with varying degrees of force, the discrepancy due to the ejection force provided by the spring of the slider cover. In IBM PC compatibles, a floppy disk may be inserted or ejected manually at any time. As the drive’s status is not continuously monitored, software may make assumptions differing from the actual state of the drive, sometimes leading to error messages or lost data.

One of the chief usability problems of the floppy disk is its vulnerability; even inside a closed plastic housing, the disk medium is highly sensitive to dust, condensation and temperature extremes. As with all magnetic storage, it is vulnerable to magnetic fields. Blank disks have been distributed with an extensive set of warnings, cautioning the user not to expose it to dangerous conditions. Disks must not be roughly treated or removed from the drive while the magnetic media is still spinning, since doing so is likely to cause damage to the disk, drive head, or stored data.

A spindle motor in the drive rotates the magnetic medium at a certain speed, while a stepper motor-operated mechanism moves the magnetic read/write head(s) along the surface of the disk. Both read and write operations require the media to be rotating and the head to contact the disk media, an action accomplished by a “disk load” solenoid. To write data, current is sent through a coil in the head as the media rotates. The head’s magnetic field aligns the magnetic particles directly below the head on the media. When the current is reversed the particles align in the opposite direction encoding the data digitally. To read data, the magnetic particles in the media induce a tiny voltage in the head coil as they pass under it. This small signal is amplified and sent to the floppy disk controller, which converts the streams of pulses from the media into data, checks it for errors, and sends it to the host computer system.

A blank “unformatted” diskette has a coating of magnetic oxide with no magnetic order to the particles. During formatting, the particles are aligned forming a pattern of magnetized tracks, each broken up into sectors, enabling the controller to properly read and write data. The tracks are concentric rings around the center, with spaces between tracks where no data is written; gaps with padding bytes are provided between the sectors and at the end of the track to allow for slight speed variations in the disk drive, and to permit better interoperability with disk drives connected to other similar systems. Each sector of data has a header that identifies the sector location on the disk. A cyclic redundancy check (CRC) is written into the sector headers and at the end of the user data so that the disk controller can detect potential errors. Some errors are soft and can be resolved by automatically re-trying the read operation; other errors are permanent and the disk controller will signal a failure to the operating system if multiple attempts to read the data still fail.

After a disk is inserted, a catch or lever at the front of the drive is manually lowered to prevent the disk from accidentally emerging, engage the spindle clamping hub, and in two-sided drives, engage the second read/write head with the media. In some 5¼-inch drives, insertion of the disk compresses and locks an ejection spring which partially ejects the disk upon opening the catch or lever. This enables a smaller concave area for the thumb and fingers to grasp the disk during removal. Newer 5¼-inch drives and all 3½-inch drives automatically engage the spindle and heads when a disk is inserted, doing the opposite with the press of the eject button. On Apple Macintosh computers with built-in floppy drives, the ejection button is replaced by software controlling an eject motor which only does so when the operating system no longer needs to access the drive. The user could drag the image of the floppy drive to the trash can on the desktop to eject the disk. The first such drives were the slim “Twiggy” drives of the late Apple Lisa. In the case of a power failure or drive malfunction, a loaded disk can removed manually by inserting a straightened paper clip into a small hole at the drive’s front panel, forcing the disk to eject manually. External 3½-inch drives from Apple were equipped with eject buttons; the button was ignored when the drive was plugged into a Mac, but not if the drive was used with an Apple II, as ProDOS did not support software-controlled ejection. Some other computer designs, such as the Commodore Amiga, monitor for a new disk continuously and have button ejection mechanisms.

Most common floppy disks in use are formatted in the FAT12 file system format, though sometimes disks may use a more exotic file system and/or be superformatted to accommodate slightly more data. Some floppy-based Linux distributions utilize such techniques.The capacity numbers given in this section assume FAT12 formatting unless otherwise noted.

Different sizes of floppy disks are fundamentally incompatible, and disks can fit only one size of drive. Drives with 3½-inch and 5¼-inch slots were available during the transition period between the sizes, but they contained two separate drive mechanisms. In addition, there are many subtle, usually software-driven incompatibilities between the two. 5¼-inch disks formatted for use with Apple II computers would be unreadable and treated as unformatted on a Commodore. As computer platforms began to form, attempts were made at interchangeability. For example, the “Superdrive” included from the Macintosh SE to the iMac could read, write and format IBM PC-format 3½-inch disks, but few IBM-compatible computers had drives that did the reverse. 8-inch, 5¼-inch and 3½-inch drives were manufactured in a variety of sizes, most to fit standardized drive bays. Alongside the common disk sizes were non-classical sizes for specialized systems.

n IBM-compatible PCs, the three densities of 3½-inch floppy disks are partially compatible. Higher density drives can read, write and format lower density media without problems, provided the correct media is used. If a disk is formatted at the wrong density, there is a large risk of data loss due to magnetic mismatch between the oxide and the drive head. Fresh disks manufactured as high density can theoretically be formatted double density only if no information has been written on the disk in high density, or the disk has been thoroughly demagnetized with a bulk eraser, the reason being that the magnetic strength of a high density record is stronger and overrides lower density, remaining on the disk and causing problems. There are people who use downformatted (EDHD→DD) or overformatted (DD→HDED) disks without problems; doing so constitutes a data risk, so the benefits (e.g. increased space or interoperability) should be weighed against the risks (data loss, permanent disk damage).

The holes on the right side of a 3½-inch disk can be altered as to make some disk drives and operating systems treat the disk as one of higher or lower density, for bidirectional compatibility or economical reasons. Possible modifications include:[14][15] Some computers, e.g. the PS/2 and Acorn Archimedes, ignored these holes altogether.[16]

* Taping or covering the excessive right holes on HD and ED to ‘downgrade’ them to DD format. This is to resolve compatibility issues with older devices using DD disks like some electronic keyboard instruments and samplers, where DD disks are useful since factories reduced manufacturing of such disks after the mid-1990s. By default, many older HD drives recognize ED disks as DD since they lack the HD-specific holes and the drives cannot detect the ED-specific hole; most DD drives handle ED and even HD disks as DD.
* Drilling or burning an HD-like hole symmetrical to the write-protect hole in order to format the disk as HD. This was a popular practice during the early 1990s, as most people switched from DD to HD for another 720 kB of disk space; special hole punchers were made to easily make this extra (square) hole in a floppy.
* Drilling or burning an HD-like hole under the ED-specific hole of an ED disk for ‘downgrading’ it to HD format. If there are many unusable ED disks due to the lack of a specific ED drive, this turns them into HD disks.
* Drilling or burning an ED-like hole over the HD-specific hole (which gets covered) of an HD disk for ‘upgrading’ it to ED format. If there are many unusable HD disks due to the lack of a specific HD drive, this turns them into ED disks.

It is possible (even if hardly officially supported on any system) to make a 3½-inch floppy disk drive to be recognized by a system as a 5¼-inch 360 kB or 1200 kB on IBM PC compatibles by changing the CMOS BIOS settings; applications include data exchange with obsolete CP/M systems like an Amstrad CPC.

The head gap of an 80-track high-density (1.2 MB in the MFM format) 5¼-inch drive is shorter than that of a 40-track double-density (360 kB) drive but can format, read and write 40-track disks well provided the controller supports double stepping or has a switch to do such a process. A blank 40-track disk formatted and written on an 80-track drive can be taken to its native drive without problems, and a disk formatted on a 40-track drive can be used on an 80-track drive. Disks written on a 40-track drive and then updated on an 80 track drive become permanently unreadable on any 40-track drives due to track width incompatibility (special “very slow” programs able to circumvent this), among other bad scenarios. Prior to those problems, there was a period when figuring out the right side of a “single sided” disk was a problem; both RadioShack and Apple used 180 kB single-sided 5¼-inch disks, labeled “single sided”, certified for use on only one side, but coated in magnetic material on both sides. The disks would work on both machines, with RadioShack TRS-80 Model I computers using one side and the Apple II machines the other, regardless available software which could make sense of the other format.
A disk notcher used to convert single-sided 5.25-inch diskettes to double-sided

In the 1980s, disk notchers were purchaseable which allowed users to punch a second write-unprotect notch in disks and thus use them as flippy disks (both sides could be used and the data storage capacity was doubled); other users made do with a hole punch or scissors and (optionally) another diskette to use as a template. The practice was particularly prevalent among home users despite warnings of damage to the disk surface due to the medium rotating opposite its intended direction. Flippy disks were adopted by some manufacturers, with a few programs being sold in this medium. They were used for software distribution on systems that could be used with 40- and 80-track drives but could not read 40-track disks in an 80 track drive. The practice faded with the increased use of double-sided drives.

Floppy disk size is often referred to in inches, even in countries using metric and though the size is defined in metric. The ANSI specification of 3½-inch disks is entitled in part “90 mm (3.5 in)” though 90 mm is closer to 3.54 inches. Formatted capacities are generally set in terms of kilobytes and megabytes.

Formatted Storage Capacity is total size of all sectors on the disk:

* For 8-inch see Table of 8-inch floppy formats IBM 8-inch formats. Spare, hidden and otherwise reserved sectors are included in this number.
* For 5¼- and 3½-inch capacities quoted are from subsystem or system vendor statements.

Marketed Capacity is the capacity, typically unformatted, by the original media OEM vendor or in the case of IBM media, the first OEM thereafter. Other formats may get more or less capacity from the same drives and disks.

Data is generally written to floppy disks in sectors (angular blocks) and tracks (concentric rings at a constant radius). For example, the HD format of 3½-inch floppy disks uses 512 bytes per sector, 18 sectors per track, 80 tracks per side and two sides, for a total of 1,474,560 bytes per disk.  Some disk controllers can vary these parameters at the user’s request, increasing storage on the disk, although they may not be able to be read on machines with other controllers. For example, Microsoft applications were often distributed on 3½-inch 1.68 MB DMF disks formatted with 21 sectors instead of 18; they could still be recognized by a standard controller. On the IBM PC, MSX and most other microcomputer platforms, disks were written using a Constant Angular Velocity (CAV) format, with the disk spinning at a constant speed and the sectors hold the same amount of information on each track regardless of radial location.

This was not the most efficient way to use the disk surface with available drive electronics; because the sectors have constant angular size, the 512 bytes in each sector are compressed more near the disk’s center. A more space-efficient technique would be to increase the number of sectors per track toward the outer edge of the disk, from 18 to 30 for instance, thereby keeping constant the amount of physical disk space used for storing each sector; an example is zone bit recording. Apple implemented this in early Macintosh computers by spinning the disk slower when the head was at the edge, while maintaining the data rate, allowing 400 kB of storage per side and an extra 160 kB on a double-sided disk.  This higher capacity came with a disadvantage: the format used a unique drive mechanism and control circuitry, meaning that Mac disks could not be read on other computers. Apple eventually reverted to constant angular velocity on HD floppy disks with their later machines, still unique to Apple as they supported the older variable-speed formats.

Disk formatting is usually done by a utility program supplied by the computer OS manufacturer; generally, it sets up a file storage directory system on the disk, and initializes its sectors and tracks. Areas of the disk unusable for storage due to flaws can be locked (marked as “bad sectors”) so that the operating system does not attempt to use them. This was time consuming so many environments had quick formatting which skipped the error checking process. When floppy disks were often used, disks pre-formatted for popular computers were sold. A formatted floppy disk does not include the sector and track headings of an unformatted disk; the difference in storage between them depends on the drive’s application. Floppy disk drive and media manufacturers specify the unformatted capacity (for example, 2 MB for a standard 3½-inch HD floppy). It is implied that this should not be exceeded, since doing so will most likely result in performance problems. DMF was introduced permitting 1.68 MB to fit onto an otherwise standard 3½-inch disk; utilities then appeared allowing disks to be formatted as such.

Mixtures of decimal prefixes and binary sector sizes require care to properly calculate total capacity. Whereas semiconductor memory naturally favors powers of two (size doubles each time an address pin is added to the integrated circuit), the capacity of a disk drive is the product of sector size, sectors per track, tracks per side and sides (which in hard disk drives can be greater than 2). Although other sector sizes have been known in the past, formatted sector sizes are now almost always set to powers of two (256 bytes, 512 bytes, etc.), and in some cases, disk capacity is calculated as multiples of the sector size rather than in just bytes, leading to a combination of decimal multiples of sectors and binary sector sizes. For example, 1.44 MB 3½-inch HD disks have the “M” prefix peculiar to their context, coming from their capacity of 2880 512-byte sectors (1,440 kB), inconsistent with either a decimal megabyte nor a binary mebibyte (MiB). Hence, these disks hold 1.47 MB or 1.41 MiB. Usable data capacity is a function of the disk format used, which in turn is determined by the FDD controller and its settings. Differences between such formats can result in capacities ranging from approximately 1300 kB up to 1760 kB (1.80 MB) on a “standard” 3½-inch High Density floppy (and even up to near 2 MB with utilities like 2MGUI). The highest capacity techniques require much tighter matching of drive head geometry between drives, something not always possible and unreliable. For example, the LS-240 drive supports a 32 MB capacity on standard 3½-inch HD disks, but it is, however, a write-once technique, and requires its own drive.

Before overhead processing, 3½-inch HD floppy drives typically have a maximum transfer rate of 1000 kb/s, with a 1× CD 1.2 times as fast at maximum and a 1× DVD 11 times as fast. While the floppy disk data transfer rate cannot easily be changed, overall performance can be improved by optimizing drive access times, shortening some BIOS introduced delays (especially on IBM PC compatible platforms), and changing the sector:shift parameter of a disk. Because of overhead and these additional delays, the average sequential read speed is 30–70 kB/s instead of 125 kB/s. Double-sided extended-density (DSED) 3½-inch floppy disks, introduced by Toshiba in 1987 and adopted by IBM on the PS/2 in 1994, doubled the number of sectors per track, thereby providing double the data rate and capacity of conventional DSHD 3½-inch drives. Some USB floppy drives use caching to increase performance while being built from standard speed drives; the X10 accelerated floppy drive was an attempt to physically increase floppy performance.

Historical sequence of floppy disk formats
Disk format Year introduced Formatted Storage capacity Marketed capacity
8-inch: IBM 23FD (read-only) 1971 79.7 kB ?
8-inch: Memorex 650 1972 179 kB 1.5 megabit [unformatted]
8-inch: SSSDIBM 33FD / Shugart 901 1973 237.25 kB 3.1 Mbits unformatted
8-inch: DSSDIBM 43FD / Shugart 850 1976 500.5 kB 6.2 Mbits unformatted
5¼-inch (35 track) Shugart SA 400 1976 87.5 kB 110 kB
8-inch DSDDIBM 53FD / Shugart 850 1977 980 kB (CP/M) – 1200 kB (MS-DOS FAT) 1.2 MB
5¼-inch DD 1978 360 or 800 kB 360 kB
5¼-inch Apple Disk II (Pre-DOS 3.3) 1978 113.75 kB (256 byte sectors, 13 sectors/track, 35 tracks) 113 kB
5¼-inch Apple Disk II (DOS 3.3) 1980 140 kB (256 byte sectors, 16 sectors/track, 35 tracks) 140 kB
3½-inch HP single sided 1982 256×16×70 = 280 kB 264 kB
3-inch 1982 360 kB 125 kB (SS/SD),500 kB (DS/DD)
3½-inch (DD at release) 1983 720 kB (400 SS, 800 DS on Macintosh, 880 DS on Amiga) 1 MB
5¼-inch QD 720 kB 720 kB
5¼-inch HD 1982 1155 kB 1.2 MB
3-inch DD 1984 720 kB ?
3-inch Mitsumi Quick Disk 1985 128 to 256 kB ?
2-inch 1989 720 kB ?
2½-inch 1986 ? ?
5¼-inch Perpendicular 1986 10 MB ?
3½-inch HD 1987 1440 kB 1.44 MB (2.0 MB unformatted)
3½-inch ED 1987 2880 kB 2.88 MB
3½-inch Floptical (LS) 1991 21000 kB 21 MB
3½-inch LS-120 1996 120.375 MB 120 MB
3½-inch LS-240 1997 240.75 MB 240 MB
3½-inch HiFD 1998/99 150/200 MB 150/200 MB
Abbreviations: SD = Single Density; DD = Double Density; QD = Quad Density; HD = High Density; ED = Extended Density; LS = Laser Servo; HiFD = High capacity Floppy Disk; SS = Single Sided; DS = Double Sided
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