And most don't charge if they can't recover it.
Most perform free evaluations so you will know more about the problem and the cost of recovery before you're obligated for anything. I haven't seen an update from Sean8832, but if you still need help and decided it's worth professional recovery, you should check a few places out and see what you are comfortable with. It is certainly possible that firmware may be involved, or some other non-physical problem, and a professional recovery company would have the tools and experience to remedy that in short order and probably with a low-end cost. Natiku mentioned the possibility of a firmware problem, and Brendan pointed out the 2TB issue too. A recovery pro can swap with a compatible board and move the necessary components to get it to work over SATA.
Some external drives don't allow for this though with USB interface built in.
:-)It's best if you can use it over SATA because some problems can kill access over USB but not SATA. Adding on to what the OP has mentioned, does it matter if the hard drive is mounted via USB or is it best to remove the casing and go SATA instead?