A word of caution to those who try to use the old HP 3-1/2" disks (i.e., the
ones that don't have springs; you have to open and close the shutter by hand)
in the 800K Macintosh and UniDisk 3.5 drives.
An old HP disk has a protective door which locks in the open position when it
is opened all the way. An Apple 400K drive doesn't slide the door all the way
open, and so it doesn't lock in the open position.
However, an Apple 800K drive does slide the door all the way open, and
because the protective door locks, the drive isn't able to eject the disk.