(This is also the reason non-Mac computers can’t mount 400K and 800K Mac floppy disks.) These drives had a variable speed motor that allowed the Mac to pack 400K into a disk that would only hold 360 KB on a fixed-speed drive. The original Mac floppy disk format was MFS, for Macintosh File System, and it is only used for 400K single-sided floppies – the only kind of floppy drive supported by the original Macintosh, the Mac 512K Fat Mac, and the Lisa 2 (a.k.a. Earlier today in the Apple Macintosh Enthusiasts Facebook group, Charles Lott asked if an OS X Mac with a USB floppy drive could write disks that a Mac running System 7 could use.
