Try with a different USB cable and a different USB port on the computer.
If that doesn't fix it, try with a different computer.
If it still doesn't work, try running chkdsk. Go to Start > All Programs > Accessories, right-click on Command Prompt and select Run as administrator. In the cmd window that appears type this: chkdsk E: /f (if your external drive is F use F there instead of E, etc.)