The factory fitted drive in my AT A4000T was a standard PC Sony drive.
The ribbon cable had the centre twist to make it unit 0. Also the ready and diskchange lines (2 & 34) in the ribbon cable were swapped.
If your faulty drive is the same, then you can plug it into a PC to test it. Note that a lot of PC drives just won't work in this configuration, so don't expect to chuck any drive in there and get results.
As the others mentioned, you can only ever use double density disks with these drives and the Amiga, even though it works in HD mode the PC.
The ribbon cable is another likely possibility, but I don't think the disks module would cause this problem, unless it was somehow not seated correctly on the motherboard.
