Cannot mount USB hard drive

There's still several elements you may not have checked. First, swap cables to make sure the cable is ok. If it still fails, then next, make sure your power supply really works. If possible, get a variable replacement AC/DC converter from an electronics store. Mine lets me set the voltage, amps, and ohms along with changing the connector tips. Use the settings on the original DC converter to test.

Assuming none of those items were the issue then you still have two other elements to check: the case and its controller and the drive itself.

To check the case: go to your local Best Buy or other electronics store and get a hard drive case. They're very inexpensive now. Pop open the old case, take the drive out, and put it into the new case. If it mounts, the issue was the case. If it still does not mount, it's the drive.

To really get rid of the drive, toss the drive in a fire. Nothing will be salvageable.

Barring also testing it on another Mac, that sounds like a hard drive logic board failure. Your best bet might be to source a logic board from another identical hard drive, with an IDENTICAL board model identifier (including the revision if any), and swap for yours, then see if it works.