I use apps like tree size and windirstat ( https://www.google.com/#q=windows+tree+size ) but I'm not finding any Windows 10 specific issue to date. That is, the same old restore cache is the repeating find.
BUT THIS IS A GOOD THING!!! I've lost count of the number of times System Restore has saved an OS reload. Why not let it work or if you need space, turn it off and back on?
Prior to installation, I had less than 100gb on my c: drive. Now, it's closer to 300.
I'm past the time for reversion--so, I don't think it is files from Win 8.1 (from which I upgraded) but that's one possibility. (However, I can't find them.)
--Maybe it's restore points? Hidden volumes?
When I clicked "show hidden files" in view from File Explorer, though, no hidden drives showed up.
I went through my Program Files and Documents directory. They seem about where they should be.
So, where in the world is that extra 200gb of data stored? I'm wondering what can possibly be happening here? I'd add that it only happens on my desktop, not my laptop. It's possible I clicked some setting on one and not the other.