This happened on LMDE-XFCE but I reckon it could apply to any flavour.

I had sized the / partition to 6.5Gigs [folks in IRC had said minimun 5Gigs would be suffient] and it was fine and only slooowly shrunk over months.  Then all of a sudden there was only 1/4Gig remaining.  I figured it was all of those Deb updates and all the accessory software that I like to install and play with, but it wasn't.

I eventually found the problem was that I had deleted some files that ended up in root's trash.  These do not show in the user trash [icon on desktop] nor in disk usage analyzer [/root directory is apparently hidden from this app] nor by right clicking root in the file manager and clicking properties [this function is forbidden unless you run the file manager as root and it never occured to me that this would fill up] 

THE ANSWER IS:  I found this using Bleachbit (as root) under system-trash.

Alternate method is: sudo thunar, look in /root/.local/share/Trash/files/

The real solution: don't leave unattened root windows open for later hazardous use!!!

perhaps this should be otherwhere than as a tutorial, leave a comment is you have a suggestion.