Are you using a HDD caddy? It is very important to use one... It keeps the HDD locked in place at all times... I had an issue with that too... I was reluctant to get a caddy myself, but it turned out to be fix.
Also, I've noticed that using hard drives that are identical to the master drives usually don't work for some reason... But it may just be a strange experience with my Toshiba HDDs.
Check Device Manager, see if you see the hard drive labeled there...