Just a couple of ideas that may or may not be relevant for you now:
1.) Run a Trisquel LiveCD and then plug the usb hdd in and see what happens. If the same problem does not exist then it is probably some change you've made.
2.) What settings are in the BIOS related to USB? 3.) Is this USB 3.0 or 2.0?4.) Is there anything on the hdd? What filesystem does it have? What partitions?
Sorry if this isn't relevant--just some random thoughts on removable hd problems.
