Got a 5400.6 ST9160314ASG FW: 0004APM2 here with Apple logo. Data is not hugely important, but would be nice to get it back.

Drive spins up and makes an attempt to read SA, after a few tries it spins down and BIOS sees it as ST_M13FQBL and 4114MB (the usual). I tried to communicate with it through terminal but get no response. Took off PCB and tried again, but still no response. I have a 5400.6 500GB drive here and the PCBs are identical. Communication with that PCB works fine.
Do these Apple drives have some weird firmware or am I looking at a dead PCB here?

Heads are almost certainly bad

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If those are Apple drives, I would be prepared for media issues also.

There was indeed media damage. Very light, not enough to damage the new heads, but spread over the surface. Swapped heads with drive of exact site/date codes anyway, unfortunately it did not work Heads swapped back into donor and that one is running fine again...

Tried serial comm. with working drive, but that one also does not respond. Apple must have done something to the firmware. Think continuing with this project will not get anywhere, as there will be extensive areas with unreadable sectors.
Case closed, data gone.