Hard Disk Space Disappeared

[iwick]

My free hard disk space has disappeared and hence my computer will barely function.

I have a 500 GB hard drive. I believe there are 4 (non-hidden) folders that should take up ~100% of the used space. These are (and current size of each - just confirmed again):

System - 5 GB
Library - 63GB
Applications - 29 GB
Users - 287 GB

Total used viewable space: 384GB

Last night I had this much on my hard drive and hence it showed me ~116GB free.

But now Finder is telling me that 492 GB are used, leaving me only 8 GB. ???

I ran Disk Inventory X and it showed me 492 GB used, but the table showing me the files only added up to the original 384GB again.

So clearly there is a HUGE file or files in a hidden folder - how do I find it? This happened once before and it was the fault of Super Duper - my backup program. There was an error in the backup and there was a huge file (drive image) sitting in the hidden folder '/volumes' - I checked that this time and there is no additional file there.

I don't think Super Duper ran but it could have - where is this file if that is the case? Is there a way to search hidden folders for large, aberrant files somehow?

Help please!