A little while back I noticed that I had run out of room on my backup drive. That seemed kind of weird, since I don't have a huge amount of stuff on my primary (Windows 7) drive. So I looked around to see what was up.
Long story short, I have about 150 GB of stuff on my primary drive. Several different methods confirm this. However, it turns out there's another 80 GB of mysterious dark matter there, which (apparently) gets compressed into 75 GB of backup dark matter on my backup drive. I'm flummoxed. What is this stuff? As near as I can tell, I've accounted for everything: hidden files, system files, swap files, hibernation files, etc. etc. But there's still 80 GB more. What can it possibly be?
UPDATE: Mystery solved! It was 80 GB of restore points accumulated since the day I bought the PC, because that was the default setting my machine came with. I cut the maximum down to 15 GB and that restored 65 GB of free space. Thanks, everyone!