A report on Group coded recording and Floppy disk variants

IBM DemiDiskette media and Model 341 FDD
Dysan 3¼" Flex Diskettes (P/N 802950)
A Maxell-branded 3-inch Compact Floppy Disk
A 3" floppy disk by Amstrad. This format was used by their CPC and Spectrum lines and in some systems by other manufacturers.
An Amstrad 3-inch floppy drive
3-inch diskette of Nintendo Famicom Disk Systems
3-inch diskette from Smith Corona labelled 2.8-inch for the diameter of the magnetic disc itself
2-inch video floppy from Canon
2-inch LT-1 floppy disk from Fuji
A write-notch puncher for 5 1⁄4-inch disks
The pictured chip, codenamed Paula, controls floppy access on all revisions of the Commodore Amiga as one of its many functions

Both took turnable diskettes named CE-1650F with a total capacity of 2×64 KB (128 KB) at 62,464 bytes per side (512 byte sectors, 8 sectors/track, 16 tracks (00..15), 48 tpi, 250 kbit/s, 270 rpm with GCR (4/5) recording).

- Floppy disk variants

In 1986, Sharp introduced a turnable 2.5-inch pocket disk drive solution (drives: CE-1600F, CE-140F; internally based on the FDU-250 chassis; media: CE-1650F) for their series of pocket computers with a formatted capacity of 62,464 bytes per side (2× 64 kB nominal, 16 tracks, 8 sectors/track, 512 bytes per sector, 48 tpi, 250 kbit/s, 270 rpm) with GCR (4/5) recording.

- Group coded recording
IBM DemiDiskette media and Model 341 FDD

1 related topic with Alpha

Overall
8-inch, 5¼-inch, and 3½-inch floppy disks

Floppy disk

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Obsolete type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined with a fabric that removes dust particles from the spinning disk.

Obsolete type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined with a fabric that removes dust particles from the spinning disk.

8-inch, 5¼-inch, and 3½-inch floppy disks
8-inch, 5¼-inch (full height), and 3½-inch drives
A 3½-inch floppy disk removed from its housing
8-inch floppy disk, inserted in drive, (3½-inch floppy diskette, in front, shown for scale)
3½-inch, high-density floppy diskettes with adhesive labels affixed
Imation USB floppy drive, model 01946: an external drive that accepts high-density disks
Front and rear of a retail 3½-inch and 5¼-inch floppy disk cleaning kit, as sold in Australia at retailer Big W, circa early 1990s
Different data storage media (Examples include: Flash drive, CD, Tape drive, and CompactFlash)
A floppy hardware emulator, same size as a 3½-inch drive, provides a USB interface to the user
Screenshot depicting a floppy disk as "save" icon
Inside the 8-inch floppy disk
Disk notcher converts single-sided 5¼-inch diskettes to double-sided.
Rear side of a 3½-inch floppy disk in a transparent case, showing its internal parts
The spindle motor from a 3½‑inch unit
A read-write head from a 3½‑inch unit
How the read-write head is applied on the floppy
Visualization of magnetic information on floppy disk (image recorded with CMOS-MagView)
8-inch floppy disk
A 3 1⁄2-inch floppy disk drive
A box of about 80 floppy disks together with one USB memory stick. The stick is capable of holding over 130 times as much data as the entire box of disks put together.

USB drives for 5¼-inch, 8-inch, and other-size floppy disks are rare to non-existent.

There were competing floppy disk formats, with hard- and soft-sector versions and encoding schemes such as differential Manchester encoding (DM), modified frequency modulation (MFM), M2FM and group coded recording (GCR).