Instead of /just/ the hard drive (much like having JUST a VHS tape) you also have some complete things it would slot against. An actual (if broken) VHS player or two to examine.
Having that hardware as a reference would exponentially reduce the difficulty of the task. Even if the capacitance values are completely screwed up over time there'd at least be a sort of range and general outline encoded by the type of components involved.
Modern electronics are harder; a lot of them are electronically programmable, so you'd actually rather hope to find one of the really cut-throat cheep units that are mass produced where actual mask-roms are used. Those might hold up over time. We can hope that some sort of advanced imaging system can 'de lid' the chip without resorting to a freeze and slice method, but even then it might be possible for the mask roms to be decoded. The EEproms will have probably long lost anything other than the fact that a given quantity of storage MIGHT have been used. The actual contents of their less-volatile registers may well have decayed past any sort of recognition.
For such cases hacker/collectors might have actually done some of the job; it's possible that some archivist's collection has an obscure dump of some firmware blob or update that is requisite for the task. A civilization level archaeology effort might be able to reconstruct sufficient artifacts to either reproduce a conformant unit or at least emulate one via more sophisticated methods.