I would rely on the drive properties to tell you about the actual used/unused state of your drive,(maybe just

check your Disk Management in case your system has hidden partitions).

One explanation:

http://www.howtogeek.com/123268/windows-hard-drive-wrong-capacity/

If it's running slow maybe check the restore folder and delete some of the earlier entries,(whilst it's in defrag

it should only be telling you what portion is free of fragmented files and not total free space).

My files are automatically de-fraged weekly,(so I can't check the system setup).