I seem to remember seeing a post like this a few years ago but don't remember how it was solved....my wife's laptop (Win7) is set up with a 70GB partition for the OS and another 70GB partition for document and photo storage. System drive is C:\ and storage is D:\. Both drives show as 85% full, give or take, and yet neither drive's visible contents come close to 20GB. I know, because - frustrated at not seeing anything glaringly large enough to account for such a chunk of space - I added up the sizes of all of the files and folders manually. All files and folders are unhidden that I can set that way, including using the attrib command in a CMD window.

Anyone remember what was writing/storing to the drive that was hidden from view? I think it was an easy fix.