You know the story ... you have two (or more) similar-model drives. And they may've been even built in the same week at the same plant. Yet try and exchange their PCB boards and you're out of luck. So my dumb question --and newbie apologies if this has already been covered -- is:
What are the parameters that determine PCB interchangeability for the SAME model?

BTW: I've heard that firmware tweaking -- for "conventional" spin HDDs -- has gotten so anal that every UNIT (i.e., every Serial Number) may now be unique, and hence, incompatible.

ROM

Be more specific!

ok. Adaptives inside ROM.

you should be more specific. My answer was realistic. Same family, same model, main difference in 99% of cases will be adaptives which will prevent operation if not transferred.

Some models will use internal ROM, like Samsung, WD and Fujitsu, these are not interchangeable without ensuring ROM version on PCB is the same, and even then some information is unique and even if its similar it wont work. Some cases you can modify the SA of one to accompany the specifics of the replacement PCB, but this is limited and comes with risks.

In more than 90% of cases, PCB from same family and same model donor is useable with right knowledge and tools. In many cases, minimal knowledge and tools will suffice.

So again, be more specific with exactly the PCB/HDD you are referring to in this post.