I had similar problems trying to boot off a HD greater than /dev/hdh.
Knoppix does not come with any /dev/ files beyond hdh.

1. Boot knoppix off CD and mount the USB HD with the knoppix installation on it.
2. Check to see of /dev/hdX exists - where X represents the USB hard drive.
3. If not you must create the hdX dev file using the MAKEDEV script in the dev dir.
4. You must insure that lilo.conf on the hard drive installed system has your usb drive dev file configured correctly. If not config it and then chroot to your installed system and run lilo to commit changes.
5. You must insure that /etc/fstab on your hard drive installed knoppix has your usb drive dev file specified correctly.