That's 931GB, so about 1TB.

I would do TreeSize then, make sure to run it as admin (there's an option once you open it in the top left). Have that scan all your folders on that drive, then expand the biggest folders to see what's eating all that space up. If you lookup any files that you aren't sure if they're important, usually you'll find some discussions about whether that file is critical or not (for example, if they're in c:\windows\system32). Comb through each large folder and see what you can clean-up, and that'll give you some space back hopefully, or at least show you what you're storing that's eating it all up.