Legacy External Not initializing

What do HD Sentinel and/or CrystalDiskInfo report? If your drive is a PATA model rather than SATA, then the problem may be as simple as oxidised contacts on the IDE interface, in which case you would only need to reseat the internal IDE cable. The Identify Device data from CrystalDiskInfo would be extremely useful in this regard. These data would identify the exact pin in the interface which is faulty or problematic.