Do I need to supply the 12V connection to an HDD I'm working on trying to control a hard drive with an Arduino Uno, but I'm having trouble with the power supply. From what I know, both the Molex Mate-n-Lok and the new SATA connectors both have 2 grounds, a 5V and a 12V power in. I'm wondering what the purpose of the 12V power supply is. I suspect it's used to drive the motor but I'm not sure.
Judging by my old copy of "Upgrading and repairing PCs" - which I found a quote for on Google Books - the 12V supply is for the spindle motor, and it says:


  Spindle motors, particularly on the larger form-factor drivers, can consume a great deal of 12V power.