I would also look at the ISO files integrity and the burn process to determine if the disk is good. Sometimes bad media is an issue. See the following link for more info: http://www.knoppix.net/wiki/Downloading_FAQ

One set of cheatcodes to try would be "knoppix dma" or "knoppix nodma". Which one would depend on the version of Knoppix you are using and your hardware's requirements. These usually resolve many optical drive read issues.

You can also try the "testcd" command. Look at the info on cheatcodes since it should be mentioned there.