In the Macintosh SE, the upper disk drive (the "external" drive) operates
differently from the lower "internal" drive or from any other 800K drive. The
upper drive sounds different when moving the head from track to track; it
sounds like the head moves three times when seeking.
The upper drive does work, but more slowly, sometimes a third more slowly.
The problem resides in the firmware. The software is testing for the source
drive at each read whenever the source drive is the upper drive. There have
been no fixes for the problem.