I had something very similar to this a year or so ago - and in the end, after painstaking hours, it turned out to be my Azureus application writing obscenely huge logs to the drive... not saying that's what your issue is, but it might be worth eliminating!
It wasn't easy to spot, cos when you search the local drive for large files, the logfiles are hidden / system files, so don't show up in the standard search
I will look up my notes, cos in the end, someone on here pointed me to a great app that takes a snapshot of all of your files / hard disk space that is being taken up, and it was that app itself that allowed me to first diagnose the issue (if not the cause)...
Hope it helps