I am sorry for this stupid question. I have tried Google for an hour, but lack of keywords failed to bring up any reasonable hint.

What I have done, is downloading KNOPPIX 5.1.1 and burnt it. It boots; everything okay. Now I need the .iso file back in order to share it. I think I read this before, but have a blackout. Of course, I could go through the whole process of remastering, but that's not what I want. Just insert the CD and extract bit by bit.
I have already tried on the level of bits and Bytes:
dd if=/dev/scd0 of=KNOPPIX.iso
cat /dev/scd0 > KNOPPIX.iso
730083328, 730148864 are the file sizes and none has the correct MD5-sum.

Thanks for any suggestion,

Uwe