Sounds like a SCSI termination issue to me.
All SCSI subsystems require termination on each end of the cables. There are two cable directions - one usually internal to the machine case and one external.
Logically, the adapter card always sits in the *middle* of the two cable directions.
This simple fact is what usually trips folks up. Most folks think of the adapter as being on one end of the cable.....
And, practically, this is often true. SCSI is usually used either with *only* the internal or external cable. There is a passive (resistive) terminator on the card to terminate the SCSI cable when the card happens to be at one end of the line.
Then, there is either in in-line terminator at the last device on the internal SCSI cable, or said device has a terminator on it's own PC board. Only the last device gets a terminator. Often when things are torn down and reused, those terminators get lost. Myself, I always used in-line terminators so I didn't have to worry about where it got lost to. All devices have none, and so I avoided the trap.
Now, if you have an external SCSI device, then there needs to be a terminator used on (or inside) it, or one an in-line unit used right at the device input connector.
My old Kodak 460 could be used with an in-line terminator or one could select the terminator inside the 460 by setting the SCSI ID to 'PC' (I think) instead of a SCSI address number. If you used the SCSI address number, then the 460 would not use it's internal SCSI terminator and one had to use an in-line one instead. Talk about a good way to get tripped up!
You might well be fighting this. If there is no termination inside the DSU, then the hi-speed SCSI signals will bounce up and down the line and, more often then not, be screwed up such that the device 'comes and goes' as far as the adapter seeing it.....
On top of all this, there is another termination issue. If you do have an internal teminator in the DSU, and one on the hard disk I see in that list *and* the card 'end termination' still plugged into the SCSI adapter - then you have three terminations as opposed to two and that extra one will load down the line too much and, again, things will be flaky.
The whole temination on each end of the line was the strength of the SCSI concept. It allowed for very high speeds over a very long length - 2 meters - which then allowed for a lot of storage devices to be attached (49 in total). That made for a nice server setup. However, you do need *two* terminators - one at each end of the total cabling - and *only* two. If you have less or more than *two* of the fool things, then the bus becomes so unstable as to be useless.
It was the common screwing up of the termination that killed off the SCSI bus.