user16100: 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.

user4143: 3.5" hard drives require 12V and 5V for the motor and logic respectively. SATA ones may also require 3.3V for the logic.  Most smaller laptop drives (2.5") only require 5V and 3.3V.  Some 2.5" drives also require 12V.
You will not be able to interface a SATA hard drive to an Arduino without some kind of SATA hard drive controller, which you could possibly build using a fast FPGA (in which case why would you bother with an Arduino?). For SATA-I (way out of date now) you need to generate a 1.5Gbps LVDS (Low-Voltage Differential Signal) data stream. For SATA-3.2 that is now 16Gbps. Impossible on an Arduino.
PATA (Parallel ATA) is more realistic, although you need a lot of pins since it is a 16-bit parallel interface. Timing is less critical since it is synchronous with the read / write strobes. 

user10794: 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.


16100: <p>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.</p>
 4143: <p>3.5" hard drives require 12V and 5V for the motor and logic respectively. SATA ones may also require 3.3V for the logic.  Most smaller laptop drives (2.5") only require 5V and 3.3V.  <a href="https://www.seagate.com/files/www-content/product-content/enterprise-hdd-fam/enterprise-capacity-2-5-hdd/en-us/docs/100827452b.pdf" rel="nofollow noreferrer">Some</a> 2.5" drives also require 12V.</p>

<p>You will not be able to interface a SATA hard drive to an Arduino without some kind of SATA hard drive controller, which you could possibly build using a fast FPGA (in which case why would you bother with an Arduino?). For SATA-I (way out of date now) you need to generate a 1.5Gbps LVDS (Low-Voltage Differential Signal) data stream. For SATA-3.2 that is now 16Gbps. Impossible on an Arduino.</p>

<p>PATA (Parallel ATA) is more realistic, although you need a lot of pins since it is a 16-bit parallel interface. Timing is less critical since it is synchronous with the read / write strobes. </p>
 10794: <p>Judging by my old copy of "Upgrading and repairing PCs" - which I found a quote for on <a href="https://books.google.com.au/books?id=jw7yCQAAQBAJ&amp;pg=PT901&amp;lpg=PT901&amp;dq=ATA++HDD+spindle+motor&amp;source=bl&amp;ots=Jvb80oa6A1&amp;sig=9H1agBSU7sXF_vYtrLlz1f16M14&amp;hl=en&amp;sa=X&amp;ved=0ahUKEwisv5aG6eXOAhVGoJQKHQU-CoEQ6AEIQDAI#v=onepage&amp;q=ATA%20%20HDD%20spindle%20motor&amp;f=false" rel="noreferrer">Google Books</a> - the 12V supply is for the spindle motor, and it says:</p>

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