If your hard drive is not listed as a storage device, try the following steps, in order.
1. Make sure the hard drive is fully inserted into the hard drive bay and looks level. Turn the console off, then on again.
2. Turn the console off and remove the hard drive. Turn the console on and then off again. Reattach the hard drive.
3. Try the hard drive on another console, if one is available, or try another hard drive on your console to pinpoint the problem.
Hopefully this solves your HDD-related problems!