Disk drive space showing full when it shouldn't be

[deikan]

Hey guys,
I'm having storage space problems with my disk space. My E: Drive is a primary partition of 638gb there are less than 100gb of files on the drive but it shows the space being less than 21gb.

I used spacesniffer and it turns out only 75.3gb of memory is actually being used for files. The remaining 541.3gb is unaccessible space according to spacesniffer.

My question is what is this unaccessible space and how do I recover it?

 

[CRITICALThinker]

You could delete your system restore points, and then make another restore point right after, otherwise someone else will have to help you with your solution.