Hi Utu
Thank you so much for all your patience. Now I think I understand.

I had assumed that the 'testcd' cheatcode just did a sha1sum test on all the DVD files - with the only difference from a manual sha1sum test being that the 'testcd' command exited with the message 'Do you want to continue anyway?' as soon as it came across the first "bad" file and so didn't check the others. One strike and you're out.

I do take your point that even if all the files on the DVD passed their sha1sum tests that would only show it was the same as the version the DVD maker used to burn the DVD.

So - what I need to do is to buy a dvd of Knoppix 7.1 or whatever the freely released version turns out to be [when that's available]. Until then I will use the least bad one I have and hope for the best.

Best wishes, john