First as jap0nes suggests try if the pendrive works correctly in another computer, if it does then try the following on your computer:
Everything you delete in the registry is gone forever so before you do anything:
For safety make a Restore Point you can restore in case you accidentally deletes the wrong key in the registry.
Access the registry (run Regedit from Start --> Run) and go to:
In the right window delete all keys that starts with ?? and #.
Delete all DosDevices keys having driveletters you don't use.
Delete the key for the driverletter you have problems with.
Close Regedit and restart your computer.
Try if the pendrive works correctly now.
Why is backslash disabled?