use a led calculator lol
use the 5v source from the hard drive
copied from Free60.org
pinout
Pin # Signal Name On XBOX 360 Signal Description
1 V33 Not connected 3.3V Power
2 V33 Not connected 3.3V Power
3 V33 Not connected 3.3V Power, Pre-charge, 2nd mate
4 Ground Connected 1st Mate, Pre-charge, 2nd mate
5 Ground Connected 2nd Mate
6 Ground Connected 3rd Mate
7 V5 Connected 5V Power
8 V5 Connected 5V Power
9 V5 Connected 5V Power
10 Ground Connected 2nd Mate
11 Reserved Not connected -
12 Ground Connected 1st Mate
13 12V Not connected 1st Mate, Pre-charge, 2nd mate
14 12V Not connected 2nd Mate
15 12V Not connected 3rd Mate
This (probably) explains why normal 3.5" sata drives won't even spin up (missing 12V). So if you want to use a 3.5" drive you need to connect your own 12V.
This table plus more info can be found in the electrical specification here . (Table 17, Page 117)