Missing HDD space.

Screwed up RAM can result in this.
I lost about 1/2 a TB on a 2 TB drive to that.

Yeah that's also possible, had it too once (lost 200GB on 1TB HD), but then the data itself gets erased, and you have a lot of (more) free space available.

The problem is that he has 50GB too few on his total max drive size, which has magically dissapeared into nothingness. It's not the data itself that he has lost.

P.S. Did I really write "3. 50GB of back blocks", haha what a terrible double typo, I meant bad blocks offcourse!

Btw, try to get a visual drive scanner (that shows the data on the disc as a graphic), maybe you see a large chunk of bad blocks there. Indentified Bad Blocks get deducted from your total hardisk size, to avoid programs writing data there.