With the original Xbox, they used hard drive locking that made it difficult to swap out to a new one. Its been a very long time since I've done it, but I seem to remember having a hard drive powered by a normal ATX PC, then swapping the data cable over from the PC to the Xbox while it was still on to achieve a HDD upgrade (with other steps in the middle that i've all but forgot).

It was a hassle for the 360 as well, you had to plug the drive into a PC first and run a homebrew tool to "bless" the drive so that it would work with the console