@whatiwonder,
do one thing only but do it right : connect the disk to a DOS based PC that run MHDD and select the disk.
If it is stuck BSY it's not the PCB and NO, a straight PCB swap doesn't work.
If the drive comes ready and you can access data on it, it's not the HDD.
If you see PWD lit , the drive is locked.
If you see other weird things like HPA lit (that is not supposed to be) then we'll think about it later.
If it is a firmware problem there's nothing you can do as expensive equipment is required and you have to work it out on your own (assuming you can).
P.s. READ AND UNDERSTAND THE MHDD MANUAL first.

And don't forget that many MHDD commands ARE destructive so PAY ATTENTION !

If you don't feel confident or you don't want to risk, just give up. Better safe than sorry.