Well, when Jimbo was installing a new DVD player in our computer, he accidentally (?) disconnected the power supply to the hard drive. We got no activity on the hard drive, to no surprise. After trying numerous things (including 'check all the connections to make sure all the wires are plugged in securely) suggested by Kitty and others on the chatbox, with no success, Jimbo sees the power supply hanging out in space and says "I wonder what this goes to?" He finds the slot on the hard drive, plugs it in, and voila, it works just fine.
Don't know if that would work for you, and it sounds like you've already replaced it, but now before we chuck anything, you can bet we'll be checking to make sure all the wires are plugged in.