If you ever spoil* your knoppix-data.img, delete knoppix-data.img; move update.tar.gz from the mnt-system into the same directory that knoppix-data.img was in and reboot;
Before this step you have to boot Knoppix with cheatcode "knoppix noimage" to avoid problems with persistent memory.

At next boottime (without any cheatcode) I run in trouble. Knoppix recognize the file update.tar.gz and uncompress it and also ask for new persistent memory. But it doesn't start to LXDE. I can only change to a terminal and reboot. I didn't find a explanation for this curious behaviour.

Greetings Werner * http://www.wp-schulz.de/knoppix/summary.html
Own Rescue-CD with Knoppix (Knoppix V6.7.1 remaster)