Zero KB available after SuperDuper crash

On Monday night, SuperDuper attempted to do two jobs at once, and got itself into a state that finally crashed. As soon as the application closed I got a message saying that the startup disk is full. Sure enough, when I got info on the HD, there was zero kb available - a couple hours earlier I checked this and there was around 6gb free on a 55gb hard drive in my Intel CD Mini.

Since then, although the system works, I can't obviously do very much - I can't save anything, copy anything to my external HD, pretty much anything except surf the web (and even that not fully - Firefox has no back button available, although the backspace key still works!)

I've ordered a 200gb HD that should be here in the next day or so, but in the meantime is there any way of regaining the lost space?

I tried running Disk Utility from the Leopard disc when re-booting, and that gained me all of 24mb 😀 - is that the equivalent of re-booting a Windows machine with the XP disc in and doing a full Windows repair, or is there another step I can try?

I'm loath to do anything too tricky, as my backup disappeared when SuperDuper crashed out, so don't want to lose all my pics etc.

Any ideas?

UPDATE:

I've managed to gain a couple hundred MB of space, downloaded Omnidisksweeper to see if it will tell me anything, but can't install it - I can't drag and drop it to my applications folder 😕

I left Mail going for about 10 minutes, it downloaded all it had from my various mail accounts, and now I have my toolbar entries and back buttons in Firefox, copy and paste has re-appeared and is happily copying my iPhoto library to a back up external HD (*phew!*)

Still only got 262mb free on the HD, but Omnidisksweeper has installed and is currently chuntering through the HD, so we'll see what it finds.

WhatSize and OmniDiskSweeper agree with each other - 38.7Gb used of a 55Gb hard drive, but I'm only showing 252Mb available.

Very strange

Found it! A sudo call told me that /Volumes had 11Gb in it - logged in as Admin, allowed myself permission, and there was a backups folder with all that data in it - deleted and back to normal.