on the computer trying to boot into linux , is it a slightly older model cd-rom drive ?
older in age cd-roms will have some more difficulty reading newer cd-rw discs

on the computer burning the "to be booted" linux disc, is it a dvd writer or a cdrw writer
i've not had any luck with the three dvd burners i have access to to get a cd burnt correctly
it doesn't work properly. explains the not finding the knoppix file, it's not reaable for some reason.
i've had this with music cd's, linux boot cds ....
after using a cd-rw, and slow, it works fine