If i am not mistaken, the 12V is the line that runs the motors, and the 5V runs the logic. its been awhile, i cant remember exactly, but this is the case for CD-ROM drives. so i dont see why it wouldnt be for hard drives.

If you have to, it may take a couple of ammeters on each supply line. But i dont think you need to go that drastic.

then look at the label of the drive, somewhere on there might be the total power consumption. Once you find this, youll know within what limits your looking at. for setting ammeter scales, etc..

Edit: If its not drawing anything at all on the 12v line, either the drive logic hasnt commanded the motor to startup, or the power supply is producing insufficient current.