Windows 7: Unknown files and/or folders taking up over 500 GB

I use a program called Cobian to back up everything from my C drive (1.5 TB) to an external (2 TB). When I backed up last night, I foolishly decided to do a mirror (though a safe one). When I got back on my computer this morning, the hard drive was nearly full. It showed there were only a couple MB left on my C drive. Before I did the last backup, there was nearly 400 GB of space remaining. I deleted a few things to give myself about 16 extra GB, but this is still frustrating me. I ran WinDirStat and it said there are 513.4 GB in <unknown>.



I've attached a screenshot of the findings. I've already tried looking around in safe mode and couldn't find anything. I am not absolutely positive doing the safe mirror operation with Cobian caused this, but I doubt it's a coincidence. I've already checked in SystemVolumeInformation, BTW. In case it matters, this stuff does not clog up the external hard drive.

I've been messing around with this for a quite a few hours and still can't find the culprit. Thanks in advance for any help.