Hi,
I have several ancients disks to test and try (>20)
2 Quantum Maverick Prodrive 540MB - One is ok and the other no dectect, so I change the PCB from one to another and the "no detect " the same behavior.

Another set of disks:
3 Quantum Fireball ST 2.1AT P/N ST21A013 Rev 02-C

One is ok the other 2 "no detect" change the PCB and the same thing.

Thanks for the answers,
Ruy Benton

PCB swap is no solution if the drive(s) have firmware issue, and relevant part of the FW is on the platters not on the PCB.