When you replace the new drive, and before you close up the case again, make sure that the ribbon connector is pushed firmly onto the correct pins on the HD. Check that the jumpers on the drive (if it has any set) are set to Master. Also make sure that the other end of the ribbon connector is pushed well onto the socket on the main circuit board. You may find that the drive isn't seen if the connections aren't correct. Best to check before you put all those screws back in!