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Have you restarted the PC since trying to run defrag? It could be that a problem has been detected and CHKDSK needs to run. Has CHKDSK actually run yet?
If you open a command prompt window (just run cmd.exe), you should perform a check to see if the drive is dirty:
If it is, I'd run CHKDSK /R (which will take a long time!) and see if there are any bad sectors or other problems. If it reports back with errors, do let us know.
Once you've cleared the drive from being marked as dirty or performed the pending check, defrag should run. It's not a good idea to cancel the disk check from running before defragmenting the drive, as it could compound a problem.