Twiggy floppy

The Twiggy floppy (officially FileWare) was a nonstandard variety of 5.25" floppy disk used on the Apple Lisa computer in the early 1980s. Apple originally had plans for release of drives for this format for the Apple II line and for the (then-upcoming) Macintosh, but ultimately went in the direction of 3.5" disks instead, leaving the Twiggy as an orphan format.

The Twiggy used media similar to the later 1.2 MB high-density disk format used on PCs, but with the casing holes arranged differently. It used a track pitch of 62.5 TPI, different from the common 48 and 96 TPI used in other formats. The drives were double-sided, and had a capacity of 871,424 bytes per disk. The speed varied from 218 to 320 RPM depending on which track was being read, allowing a near-constant amount of data per inch.

The disks had 1702 sectors storing 512 data bytes each (with 20 additional tag bytes per sector). Storage used GCR encoding.

Links

 * Wikipedia article
 * Info on Twiggy floppy
 * Anecdotal story
 * Apple patent on Twiggy drive
 * Rare prototype Mac with Twiggy drive
 * Lisa OS goes open source