I have a couple of Western Digital 500GB WD5000LUCT disks pulled from set top boxes, unfortunately these run a specific firmware that prevents use elsewhere.
I don't care for the data that may be on the disks but would like to be able to use them in laptops or for backups, I've identified the WD5000LPLX and WD5000LPCX as being at least very physically similar.
Would it be as simple as getting the right "standard" firmware onto the SPI Flash or is it likely to have more twists in it than that?
Any chances of screwing the magnetic side of the disk control for good by trying the wrong FW?
This is unlikely to work.
For once, just because they appear similar from the outside doesn't mean the drives use the same internals (the housing/chassis is often common across models). The WD5000LUCT is an AV Series drive with 5400rpm specifically aimed at DVR use, and quite often the hardware is very specific (very low power, very low noise, focus on stable sustained data rates instead of raw performance, and sometimes even with lack of support of certain SATA features). The WD5000LPCX is a 5400rpm desktop drive of the WD Blue Series and the WD5000LPLX is WD Black Series desktop drive with 7200rpm, both which are unlikely to use the same chipsets as the AV Series.
The other thing is that on may modern drives not always is the firmware in a ROM, but often parts or even all of it might be located on the magnetic media (the ROM then only contains software to initiate the firmware download from the platters).
Lastly, your issue might not be the firmware (the AV Series can, after all, be used as regular SATA drives), it's more likely that the drives have been firmware locked, which is a common thing on many DVR platforms to prevent people from directly accessing recordings,. thereby circumventing the settop box DRM.
If you're really sure it's a firmware issue then I would suggest looking for the AV Series firmware for that drive.