Floppy Disk

diskette · flexible disk · IBM 23FD · Minnow

IBM's own engineering history shows how much changed within five years: the 1971 23FD read at 90 rpm with FM encoding, the 33FD (announced 1971, shipped 1973) added write capability at 360 rpm, and the 43FD (1976) used both sides for about 0.5 MB.

Glossary entry on hesela.dev · JSON record

In one minute

The floppy disk is a removable flexible magnetic disk sealed in a jacket, first shipped by IBM in 1971 as the read-only 8-inch 23FD diskette drive (code name Minnow) that held about 80 KB, roughly 3,000 punched cards, and was built to load microcode into IBM mainframe controllers.

IBM's engineers did not start with a disk. In 1967 D. L. Noble evaluated rigid disk files, magnetic belts, dictating-machine disks, a stretched-membrane file, 45 rpm phonograph records, and audio tape cartridges for a cheap way to load control programs, found none satisfactory, and proposed a removable flexible disk (IBM Journal of R&D, 1981). IBM's history page and the Computer History Museum both date the first sales to 1971.

The first design was read-only. The 23FD spun the disk at 90 rpm, wrote 32 tracks at about 1,100 bits per inch that rose to 1,594 bpi (81.6 KB formatted), and used self-clocking FM encoding with a head pressed against the disk. Because factory writers wrote the disks, IBM wrote wide and read narrow to tolerate drive-to-drive misalignment. A 1969 experiment with modified frequency modulation (MFM) doubled capacity on paper but was dropped for excess bit shift.

The 33FD, first included in IBM announcements in 1971 and shipped in 1973, was the read/write version. It quadrupled speed to 360 rpm, raised density to 3,268 bpi, used a phase-locked oscillator for the 250 kbps data rate (7.5 times the 23FD), replaced fixed sector holes with an index hole and recorded sector markers (soft sectoring), and stored 242,944 bytes on 73 tracks of 26 sectors. In 1976 the 43FD added a second head for about 0.5 MB.

Outside IBM, the Computer History Museum records that Alan Shugart left for Memorex, which introduced a read/write 8-inch drive in 1972 with a 175 KB disk, and then founded Shugart Associates in 1973. IBM says Shugart Associates supplied the two 5.25-inch drives of the 1977 Apple II. IBM places high-density 1.2 MB disks in 1984 and the 3.5-inch 1.44 MB disk in 1986, while the museum text lists a 1.2 MB 8-inch format in 1977, so the 1.2 MB figure is attached to different formats in the two sources.

Compared to neighbors

A hard disk drive keeps its platters sealed in the drive and flies the head on an air film; the floppy's medium left the drive in a jacket and the head touched it. The two sources disagree on who led the project: IBM and the museum name Noble as leader and Warren Dalziel as lead inventor, while the patent names Dalziel, Jay Nilson, and Donald Wartner.

IBM diskette drive generations, from the IBM Journal of Research and Development (1981)
Drive	Shipped	Spindle speed (rpm)	Density, innermost track (bits/inch)	Data rate (kbit/s)	Capacity per disk
23FD (read-only)	1971	90	1,594	33.3	81.6 KB formatted
33FD (read/write)	1973	360	3,268	250	242,944 bytes user, 73 tracks x 26 sectors
43FD (two-sided)	1976	not stated in the text read	not stated in the text read	not stated in the text read	about 0.5 MB

Strengths

Cheap random access on removable media — IBM's engineers kept the diskette over cassette tape because of random access, plus a growing need for a common magnetic exchange medium (IBM Journal of R&D, 1981).
The jacket made it handleable — a plastic jacket lined with nonwoven fabric protected the disk and wiped it clean while it spun; the paper calls the enclosure key to wide acceptance.
Portable software and data — IBM says floppies let software companies sell programs by mail or in stores and gave users a "sneakernet" for moving files.

Limits

Tiny capacity by later standards — about 80 KB on the 1971 23FD and 242,944 bytes of user data on the 33FD, against the 1.44 MB 3.5-inch format IBM describes for 1986.
Contact recording wore media and heads — the head touched the disk, so the IBM team tested wear, head contact pressure, environment, and lubricant for the 33FD.
Exchange between drives was a tolerance problem — IBM used write-wide, read-narrow heads on the 23FD and tunnel-erase guard bands on the 33FD so that a disk written on one drive could be read on another.
Software outgrew it — IBM notes the diskette version of Windows 95 took 13 diskettes installed one at a time.

Typical fits

Loading microcode and diagnostics into IBM controllers (the original 23FD job)
Data entry in place of punched cards
Software distribution and file exchange between personal computers

Uncertainty notes

IBM's claim that more than 5 billion floppy disks were sold annually in the mid-1990s is IBM's statement, not independently verified here (vendor claim).
Capacity of the first 23FD is given as 80 KB by the Computer History Museum and 81.6 KB formatted by the IBM paper; both are shown.
The 3.5-inch introduction year comes from IBM's page only; a second primary source was not opened.
The 43FD speed and density were not read in the extracted text, so the table does not guess them.

Sources

01IBM — Floppy disk storage (IBM history) · accessed 2026-10-09
02Computer History Museum — 1971: Floppy disk loads mainframe computer data (The Storage Engine) · accessed 2026-10-09
03J. T. Engh — The IBM Diskette and Diskette Drive, IBM Journal of Research and Development 25(5), September 1981 · accessed 2026-10-09
04US patent 3,678,481 — Data storage apparatus employing a single magnetic disk (Dalziel, Nilson, Wartner; filed 13 March 1970, granted 18 July 1972) · accessed 2026-10-09