The Knoppix CD ISO should fit on a (new and empty) 700 meg CD blank without overburning. It will not fit on a 650 meg CD blank even with overburning (I think that is mentioned in the FAQ). Are you using a 700 meg blank? You should be able to confirm that the size of the media is 700 meg before burning (actually it will very likely report that it is 703 meg, all modern 700 meg media that I have seen report at that size, unlike older 650 meg that could vary somewhat). Is 8x as slow as Nero will let you go? (With some drives it is, with others you can go slower, if you can then I urge you to do so).