Hard Drive overloaded - how?

I'm actually asking this question on behalf of my father with relation to his desktop computer, but the principles are the same here.

A few weeks ago, he noticed that his half-full 1TB desktop hard drive suddenly bloated up to approximately 98% full. It's a machine that is typically used by my half brothers who have no understanding of safe computing, so they're all over the map on what they download, install, do on websites, and so on. The problem is that, even with hidden files and folders shown, nobody can track down more than about 400 GB of data on the hard drive, meaning there's about half a terabyte of hidden bloat.

My first suspicion was system restore, but it's only got 1% allocated and wasn't even taking up that much in the first place. The pagefile is quite small as well. It's been suggested that a video editing program might be creating and not cleaning caches (I think my little brother uses Sony Vegas Platinum 11.0), but I haven't found anything in appdata or that sort of thing. The recycle bin is also empty. Disk Cleanup has been run multiple times, defrag claims low fragmentation...

So, any ideas what might be invisibly taking up half a terabyte of space?

 

Is there a lot of media on the machine? Videos, songs, books, etc? I had a similar experience, once with Itunes, once with WMP where i somehow instructed each of those programs to duplicate all my media. Filled up my hard drive pretty quick. I was able to use a freeware program (cant remember) that automatically de duped the media. This is circa 2009, but it's my best guess.

 

maybe shadow copys from system back up? try some software than show disk usage
WinDirStat
treesize
folder size

 

I would boot into safe mode (disconnected from the internet) and run a full scan with an up to date AV and Malware program

 

Can't check any of that stuff for a couple days (going camping in the morning), but rep all around. I'll report further when I try it out.

 

windirstat has a portable version, you can run it from a flash drive.

WinDirStat Portable | PortableApps.com - Portable software for USB, portable and cloud drives