I've just taken the GVP SCSI HD8+ unit apart to try looking for faults and here's what I found. I'll upload and post some pics later.
I should point out that I don't know much about electronics and I'm just trying to get into it again for the third of fourth time. There was only one component which looked like a resistor. I checked it with my multimeter, but I got a read out, which was fluctuating, so obviously this was within a certain tolerance range. This means it hasn't blown out. There were other components marked as R followed by a number, but they didn't look like resistors to me. I tried and failed to get readings from them.
I unplugged the cable connecting the SCSI drive to the main board. I noticed that part of it was bent and that one or two wires may be broken. I played with it in my hands, trying to bend it in the opposite direction, in case this may bring the possibly broken wires in contact again.
After all this, I reassembled the whole unit and slotted onto the side of the A500 Plus again. This is a very delicate and frustrating job, which I feel may well break the connector on one or both units. After turning on the GVP unit, waiting for it to spin up, I turned on the A500 Plus to see what happened, but there was no change. The drive still spun, the drive light came on, but it failed to boot up. It tried to boot three times, then fell back to df0: as the boot drive. I also tried using the drive disable switch and when I did, it made no attempt to boot from hard drive, exactly as it should do. I then flicked the switch to re enable the drive and it tried unsuccessfully to boot again.
Obviously, my next step should be to get hold of a compatible cable. I'm not sure if an old PC bought in 2000 would have one fitted, but I'll check. If not, it may be a few days until I can get hold of one.