I’m not a data recovery guy, not even an amateur, so I can only try to help you with very basic things. Please don’t accept the following statements as Gospel.
That said, I believe it is safe to swap WD and Seagate PCBs without transferring the EEPROM, but the DR professionals at HDD Guru warn against trying this on certain Samsung drives, IIRC. The EEPROM stores unique “adaptive” calibration data. If the tolerances between two drives are close, then a straight swap may work on some models, otherwise the drive will click, or power down, or identify itself with its internal family name.
You may like to refer to this thread for a more detailed explanation:
A dead preamp, particularly if it presents a short circuit across its supply rail, may also kill the associated DC-DC converter on the replacement board. You can test the preamp for shorts, but I don’t know of any other way to test it other than by powering it up.
I have often seen damage extend beyond the TVS diodes, so I am not surprised that this happened to you.
I see from your latest post that a board swap has failed, although the drive does spin up now. The next step is to transfer the EEPROM. If you are not adept at soldering, then your local TV/AV repair shop should be able to do it for you.
BTW, before you do anything else, I’ll try to help you test the preamp. There appear to be three DC-DC converters near the SMOOTH chip, each with its own inductor (L4, L1, L3). We need to do a little circuit tracing to find out which one connects to the 22-pin preamp pads at J1. If you can bear with me, I’ll compose another message with additional suggestions. I’ll also try to answer your PM.
Good luck, and keep cool.