ED 3.5 Diskette Labelled As 4 MB, Formats As 800K

The Extended Density (ED) 3.5 inch floppy disk (which is labelled as 4 MB, but formats at 2.88 MB), is NOT supported on any Apple Macintosh computer as of February 1995. The floppy drive mechanism that supports this disk is primarily found in MS-DOS/Windows and UNIX workstation computers.
While holding the diskette with the shutter in the up position, you will notice two square holes on the disk. The one on the lower left is for locking, and the other on the lower right indicates the density.

.           Shutter
.              |
.              V
.   +---+--+----------+--+       +---+--+----------+--+
.   |   |  |      +-+ |   \\      |   |  |      +-+ |   \\
.   |   |  |      | | |   |      |   |  |      | | |   |
.   |   |  |      +-+ |   |      |   |  |      +-+ |   |
.   |   \\__\\_________/    |      |   \\__\\_________/    |
.   |                     |      |                     |
.   |   +-------------+   |      |   +-------------+   |
.   |   |             |   |      |   |             |   |
.   |   |             |   |      |   |             | @ |<--Extended
.   |   |             |   |      |   |             |   |   Density
.   | @ |             | @ |<-+   | @ |             |   |   hole
.   +---+-------------+---+  |   +---+-------------+---+
.     ^                      |
.     |                      |
.   Locking hole      High Density hole

HD (High Density) disks also have an ID hole. The HD disk ID hole is parallel to the locking hole. The ED ID hole is offset about a 1/4 inch closer to the shutter. Since the ID hole is in a different location, the Apple SuperDrive treats the ED disk like an 800K disk, which does not have a density hole.
Published Date: Feb 19, 2012