You aren't understanding me, if you take a HDD from ANOTHER computer and hook it up to yours to fix the one from the OTHER computer, it won't infect yours. If the other HDD was infected in another computer, it can't openly infect yours if you just hook it up externally. That's almost like hooking up an MP3 player to your computer, it won't do anything.
Now if you have a second HDD, yea it can infect that one, that is another scenario entirely. But we are saying hooking up the infected HDD to another computer that isn't infected externally. If he does that to run scans on it, the infected HDD is more or less just attached as free weight at that point and can't do anything but accept the scans.
Does Hard drives have firmware?
Does the O/S query the hard drive device driver that queries the added hard drive firmware?