Have you run a diagnostic scan on the hard drive to ensure that there is nothing physically wrong with the drive? Also, are your files backed up?? If you have enough space, you can copy the contents of the drive back to your machine, then reformat the drive and copy the files back.
I do have another solution involving a batch command called ICACLS, but I hesitate to put it out there due to the possibility that it could nuke your files (i.e. due to not knowing what caused this problem, and also due to the fact I've never tried it on an exFAT formatted drive, so I don't know how well it will work).
If you want me to post it, though, let me know and I will. Maybe the gurus here could tell you the odds of it working. For some light reading, http://technet.microsoft.com/en-us/m...alltrades.aspx
EDIT: Per a PM request, I'm posting the ICACLS command. If you have a lot of files on the drive, it can take hours and hours to run, so you might want to try it on a single file to see if it works before committing the command to the whole drive.
From an escalated command prompt, run the following command and press enter:
icacls X:\* /T /C /grant everyone:(OI)(CI)F
(where X: is the dive letter of the external drive and \* the path to all files on the drive -- i.e. replace X:\* with the drive and path/name of a single file or folder to limit the command to that file/folder).
Note: Accidentally running this command on your boot drive will almost certainly frack up your windows install, so make sure you run it on the correct drive. It is also possible to easily change permissions on a file so that nobody can open it, so be careful with it. . . .