Well, if it's a 2.5in Drive, it probably doesn't use the 12V connection.

As for the (physically) larger sizes, 12V ceratinly was for the spindle motor when I played with drives (no saying that they don't have a regulator to drop the 12v to something else nowadays - check your PCB). Going back to when I was a teenager and I used to pick up old 5.25in HDDs at the junk store for a few cents, they would have a jumper (or switches) to stagger the power-up time of the spindle motor at initial switch-on, so as not to overload the 12v line if you had multiple drives in a system.