External USB drives (aka the least reliable form of backup ever devised by man) are often corrupted. If you have no important data on it you can just go into disk management and reformat the drive.
If you have data that you trusted to the drive I would first connect to other USB ports and another computer, then try EaseUS Partition Manager from HERE
to do the quick repair to the partition. If that fails, try recovering data with Recuva.