800K floppy

A 3.5" floppy disk that holds 800K of data. First introduced on the Apple II line of computers with the UniDisk 3.5 drive. Was later used on the Macintosh. To save money, 3.5 drives used on PC's were based on CAV, so they could store only 720K of data, as opposed to 800K on the CLV 3.5 drives used on Apple machines. 800K floppies were later obsoleted by 1.44MB HD floppies.

Reading one of these may make a Mac Plus still useful, outside of a History of Computers museum

800k 3.5 inch floppies were (and still can be) used on Risc OS computers. They used a format called ADFS. ADFS was later upgraded to hold 1.6Mb, that extra space doesn't mean much nowadays when most software is measured in Megs or Gigs, but at the time a few extra K made all the difference.