Thanks kindly for the advice. What eventually got it to work was simply trying a different brand of media: Verbatim did not work with K3B & my Memorex DVD burner, but Memorex brand DVD-R's did work. I'll be buying Memorex from now on. I was having problems even though the MD5 check was good.

Given a good MD5 check a the beginning, I was ready to blame the vintage of the Knoppix image, because it failed to burn (with "verify written data" at end) or failed to boot (without "verify written data"). Silly me.