I didn't see anyone mention the "invisible on SCSI" symptom you
mentioned. Did you check whether it was HVD vs SE vs LVD scsi?
HVD is common in older drives with external connectors due to better
external noise immunity. It would not be able to hook the HVD
up to an SE, though the LVD and SE can be mixed.
when you hook an HVD device to SE there is a pin which shorts out the SE
drivers and HVD drivers so that there should be no damage,
as long as you do the connecting while the power is off.
I look at the driver farm on the controller and the target device for
clues as to which I have, also as to whether you have a real SCSI
device or just some connector which is the same as SCSI, but isn't scsi.
Jim