FDHD

FDHD refers to a floppy drive high density mechanism (also marketed as a "SuperDrive"), introduced by Apple Computer in the Macintosh IIx in 1988, and last used in the "beige" Power Macintosh G3 all-in-one in 1998.

Description
The controller for FDHD mechanisms is called the SWIM chip (an abbreviation of Super Wozniak Integrated Machine or Sander-Wozniak Integrated Machine, according to different accounts. The SWIM chip replaced the Integrated Woz Machine (IWM) for older 800 KB floppy mechanisms. It was available as part of an Apple FDHD Macintosh II Upgrade Kit (part number M0244).

The newer floppy drive mechanism could read 1.4 MB high-density floppy disks, as well as disks formatted for other operating systems. However, reading MS-DOS or ProDOS disks initially required the use of Apple File Exchange, until PC Exchange became available in 1992.

An external Apple FDHD Drive (part number G7287) was also marketed, but if used with a Macintosh or Apple II series computer without a SWIM chip in the system or controller card, would only function as a 800 KB drive.

Issues
Users found that high-density floppies that had previously been used in a 800 KB drive were unreliable when reformatted for high-density use. This was because the FDHD mechanism switched to a weaker magnetic field to write to smaller sectors, sometimes being unable to thoroughly erase data written with a stronger magnetic field from an 800 KB drive. A workaround was to use a powerful magnetic field from a to wipe such disks.