If it used to work, why not do a System Restore back to when it worked.

If that doesn't help, it's more difficult. Things to choose from:
- repair install of Windows 7 RC (if such is possible)
- clean install of Windows RC again
- clean install of Windows 7 final version (from October 22 - will be necessary anyway next year if RC ceases to function)
- other USB hardware (f.e. a new PCI-USB card) or a powered hub
- other enclosure for your disk