tannaroo, as I mentioned in the other urbackup Newbie thread, I had this same login problem. I took the following steps, based on the suggestions above to solve this. there may be easier ways of doing this, but this worked for me.
You need to boot from a different linux disk, (I booted from a linux install on a usb stick), and then mount the systemdisk, for me the original /dev/sda2 . I was then able to look around on my disk using du -h -x -d1 as suggested above. This allowed me to discover a directory in one of my sharedfolders mount points which had a lot of data, 127GB, that should not have been there. Because you do not have your other share disks mounted, the mount points in sharedfolders should have no files. When the disk was running normally those files were there taking up space, but hidden by the mounted share. I'm currently moving the files somewhere else and can see my original system disk getting less full as I move them.