Need help solving Zip Drive mystery

[supertimer@aol.com (Supertimer)]

I'm trying to help an Apple IIGS user set up a Zip drive.
The problem is that his setup is not working.

He has an Apple High Speed SCSI card.  There are
other drives on the system.  Termination is correct
and SCSI IDs are unique for each device and the card.

The hard drives on the SCSI chain show up but the
Zip disks are not recognized.  ADU momentarily shows
the unformatted Zip disk, but then progresses to the
next volume before any selection can be made.

He boots up the system with the Zip disk in the drive.
If he ejects the Zip disk at Finder and reinserts it, the
computer freezes.

Also, in open and save dialog boxes, the computer
is constantly polling his drives.  This is the same
behavior as on my IIGS with HS SCSI card when there
is NO Zip disk in the drive but the Zip drive is on.  On
my system, inserting a Zip disk stops the polling.  If
the Zip disk is blank, the computer asks "Eject or
Format" on my system.  There is no such message
on his system.

He has tried his Zip drive on another IIGS with the
same results.

Any idea what's going on?  Could the Zip drive have
been DOA?  If so, what's the typical signs?

He is using DOS formatted Zip disks.  The IIGS takes
these fine on my system and can be reformatted and
partitioned easily.  That's not the case with his system
and drive.