Hey kbmf. The guys have given you some pretty good advice, but I'd recommend that you test the drive with the manufacturer's diagnostic tool. Also you should try the basics of troubleshooting first: try the drive with a different USB cable (not longer than 12") and try it with a different computer and different USB ports (USB 2.0 and 3.0). As @fzabkar has mentioned your last resort should be, taking the drive out of the enclosure and connecting it directly to your motherboard via a SATA connection, but note that if your drive is still under warranty, this would probably void it.
Hope that helps. Good Luck.