Number of Supported Floppy Drives
The Macintosh SE has an external drive port, but the Macintosh II does
not. If two floppies are installed internally, the Macintosh SE can
support as many as three floppy drives. The Macintosh II can support no
more than two floppy drives, and cannot support an Apple HD20 or external
floppy drive.
SCSI Drive Interleave Scheme
The ROMs of the Macintosh SE and Macintosh II have different SCSI disk
interleaving schemes. Although disks are interchangeable, they will work
most efficiently on the system on which they were initialized. The
Macintosh SE uses a 2-to-1 interleaving scheme, while the Macintosh II
uses 1-to-1 interleaving.
Therefore, a SCSI disk initialized on a Macintosh SE and moved to a
Macintosh II will not transfer data as quickly as a Macintosh II SCSI
drive, since the Macintosh II will be forced to wait for every other block
of data, rather than using them in sequence.
If a Macintosh II SCSI drive is taken to a Macintosh SE, the speed
difference will be even more apparent. The Macintosh II drive will have
blocks allocated in sequence on each track. But the Macintosh SE will not
be able to accept the data that quickly, so the drive must make almost a
full rotation before the second block again passes under the head, and the
information can be read from the drive.