@olePigeon Darn. I was hoping it was related to SCSI Manager 4.3.
My understanding is that the original implementation of SCSI on the Macintosh allowed multiple SCSI hardware busses, but only one logical SCSI bus.
This is why the Q950, which has two SCSI busses (? or when the PDS card is installed?) still requires unique IDs even if the devices are on different busses.
SCSI Manager 4.3 added logic so that different physical SCSI busses are also different logically, and now SCSI busses each have their own set of unique SCSI IDs.
However, there's a gotcha. If SCSI Manager 4.3 is not in ROM (all pre-Q-AV machines, IIRC) then at boot time, the machine is going to treat all physical SCSI busses as a single logical bus and overlapping SCSI IDs on different busses may still cause issues until the SCSI Manager 4.3 extension loads.
The JackHammer gets around this (I think) by loading a copy of SCSI Manager 4.3 from its Declaration ROM.
The Turbo601 basically has PM6100 ROM on-board and it also loads a copy of SCSI Manager 4.3 because of that ROM.
That's how it is meant to work. But if one is having trouble with devices not appearing in early machines, SCSI Manager 4.3 is a good thing to look at. However, it sounds like it didn't help in this case.
I feel like there was something that solved the issue with my Tape Drive, but it may have been specific to Retrospect software.