From what I have read, your Xbox HDD needs to be powered by the Xbox so that when the computer reboots, the HDD does not lose power.
No no no...
You don't do that. You just power it from your PC. When you think of it your PC runs the same hdd as your xbox (difference in storage of course) so why bother connecting the new hdd to your xbox, it would be easier to get the power from the PC. And it doesn't matter if your new xbox hdd reboots with your PC. Because you have to start xboxhdm anyway so by the time you get to the Xboxhdm Boot-prompt your hdd is all powered up.