In case it is a software issue try the following:
Open Device Manager, scroll down to (Universal Serial Bus controller)" and uninstall all the USB Hubs and controllers , then restart the computer to let Windows reinstall all the USB ports again. Then go to the first USB hub and right click on it and go to properties. Then on the advance tab click on : "do not show warnings" or "do not tell me about usb errors" (depending on you OS)
If it is a hardware issue You better take your HDD out of its enclosure. (iomega) enclosures tend to fail more often.