(There is also a 20 MB maximum volume size for MFS hard drives.)Īll Mac OS versions up to System 7.1 support formatting 400K floppy disks. Disk access is controlled by code in the system ROMs, and that can’t be updated to support double-sided drives. There is also no support for floppy disks with greater than 400K capacity in those original three models. There is no support at all for 400K floppy disks in Mac OS 8 and beyond.

Under Mac OS 7.6 and 7.6.1, they can read 400K floppies but not write to them. Macs with 800K double-sided drives and Mac-compatible 1.4MB High Density (HD) drives can read and write the MFS format used by 400K disks as long as they are running System 7.5.5 or earlier. (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.

Format floppy disk