well id say theres a partition 4 the rest that is hidden or damaged but never happened to me b4 try right clicking my computer then manage go into storage then disk management shows you every think hope this helps
Personally, I would say you might want to run Command Prompt and run a chkdsk /f. That should fix any errors on your next boot.
If it is more than that, you might have a corrupt hard drive or, as Simon said, there could be a hidden partition. Just go to Control Panel, Administrative Tools, open up Computer Management and click on Disk Management. That will give you a list of all of your partitions plus allow the ability to tweak with them.