My seagate barracuda 7200.11 1.5TB has ~550GB of files on it, yet when i click on the drive itself it says that 900GB have been used. This has been used from the windows 7 beta to now, and has been through many system refreshes without formatting so that may be the cause. I have a 2tb external HDD so re-formatting is not out of the picture. I can provide photos of the properties and all files if needed. I am running Windows 8 Pro.
Defrag it?
Just tried that, still says 873gb used.