I have a Hitachi Deskstar 250GB hard drive that stopped working(dead). I bought an identical replacement(that works) then I swapped the circuit board out as instructed in many videos. I plugged it in the computer and now the hard drive has power to it(hums) but the BIOS doesn't detect the drive. Any idea why?
Most modern HDDs store unique, drive specific "adaptive" information in a serial EEPROM chip. This chip, or its contents, needs to be transferred from patient to donor. In the case of Hitachi drives, this chip is referred to as the "NVRAM".
The following PCB suppliers offer a firmware transfer service, either for free, or for US$10:
I would advise that you avoid those suppliers who don't tell you that a board won't work without modification. Often they will attempt to obscure the requirement for a firmware transfer by deceptively describing their products as being "for data recovery only".
Alternatively, if you are not adept at soldering, your local TV/AV repair shop should be able to transfer the chip for you.
Some PCBs do not have a discrete serial flash memory chip. Instead they store the adaptive data inside the Marvell MCU (the largest chip). In this case you will need a "PCB adaptation" service.