Quick One! Clear Phantom Hard-Disk Hogs.
If you use Adobe Bridge you might want to change your cache settings. Every time you view a large file you are most likely viewing a preview of that file generated by bridge. Bridge saves these previews in the cache so that next time you view the file, it doesn't have to waste time generating a preview. The problem is that this can amass many gigabytes of data in a rather short while.
Open Bridge, press Ctrl+K (Win) /Command+K (Mac) and under 'Cache' you can start by changing the location of the cache to a partition of your drive that has more space. If you are bold enough you could even purge the cache altogether. Either way, you now know where those phantom 20 gigs have been hiding.
One more thing, if you've ever used (or are still using) Bridge CS3, you might want to search your system for a file named 'filesystem_blobs.MYD'.