I don't know much of the way things work here, but you can probably use: strace -o sulog.txt su - [the final - is optional - man su] Then read sulog.txt, it might give a clue as to what it was trying to do. strace is in the strace package - if you can't install because you're not root you can probably do something useful with the source package - apt-get source strace Also, have you tried booting to single-user mode? Good luck.