I rebooted with the knoppix vga=0 debug -b 3 command, got:

Compressed KNOPPIX image is broken: /mnt-system/KNOPPIX/KNOPPIX on /dev/sr0 on /mnt-system type iso9660 (ro,reltime)
[ 6.566185] cloop: Bad file, read() of first 4096 bytes returned -5.
could not mount /mnt-system/KNOPPIX/KNOPPIX. Starting debug shell...
/sh: can't access tty: job control turned off
/# past mounting /UNIONFS.
/sh: can't access tty: job control turned off
/# Past symslinks to /UNIONFS
/init: line 973: /bin/busybox: not found
/# init: line 973: reboot: not found

I am sure the disk works because I was able to boot another computer with it.