External HDD unrecognized

You've probably tried a lot of this, but I'm going to go from the ground up to make sure nothing gets miss.

1) Make sure the port works. Plug in another USB device, preferably something with storage so you can test transfers.

2) Test or replace the USB cable to rule it out.

3) Look in your device manager. (Control Panel > System and Security > Device Manager). This is probably where you'll find the device doing something odd.

I suspect the interface board on your external drive is bad, but the only sure way to test that is take the drive out of the enclosure and connect it to a known-good controller.