if it's an external HDD you might try checking if it's not the drive but the controller inside the case. That happened to me one. The controller failed, so I purchased a new drive and (as then I didn't know) tried recover the damaged hard drive just to try, removing the new one and attaching the damaged one to the new controller. (inside the case there was just a normal HDD attached to a small circuit board) so after replacing it, the computer recognised it. so I just got that hard drive connected as a normal internal drive...