there are other ways to power a HDD than a pc's PSU..

there are such a device as variable voltage PSU's..

12.3 volts sounds a bit sus.. as that is with in the 10% that many (cheap) PSU's work with in.. the good PSU's are better than 5%.. so the .3v is still with in the 5%..

certainly you could set up a test bench with a fixed 5v and a variable 12 source..

If there is a problem with the HDD motor.. a voltage boost ONLY on the cotroller output could bring a temporary life to the drive.. most reliable is as mentioned.. clean room, remove platters..

Sick motors kill more controllers than anything else..