ST2000LM003 weird power issue - need some clarity

[murphman]

A customer brought in an older LaCie Portable Rugged RAID; inside are 2x ST2000LM003 drives.

Both drives do not spin up. One drive shows no LED activity at all and the other stays in BSY (but still doesn't spin).

Here is the strange thing.. when I swapped the PCB between both failed drives, the one that had no power (and no led) just started fully working, no issues!!! the other drive still has the original problem, doesnt spin up and stays in BSY. I tried putting back the original PCB (now that i know it works on the other drive) and even tried other donor PCB but still the same issue, doesnt spin and stays in BSY.

I am assuming its a preamp on the heads (what else could it be if the PCB is good and the motor isnt jammed?).

What's bothering me is how can a drive with a FUNTIONAL PCB have a bad preamp while its RAID sister drive just has a fried PCB? Just weird.. I havent tried swapping the heads yet but I can only assume its the preamp, if anyone got any other suggestion or thoughts on what it could be, I am all ears. thanks!