Thanks for the suggestions, however my problem is not solved.
Neither the option "fromhd=/dev/uba1" or "fromhd=/dev/sda1" works any better.
I am sure the linuxrc script tries both devices by default, but fails to mount them.

from linuxrc (in minirt26.gz):
# New: also check HD partitions for a KNOPPIX/KNOPPIX image
test -n "$FOUND_SCSI" -a -z "$NOSCSI" && DEVICES="$DEVICES /dev/sd?[1-9] /dev/sd?[1-9][0-9]"
DEVICES="$DEVICES /dev/ub?[1-9] /dev/ub?[1-9][0-9] /dev/hd?[1-9] /dev/hd?[1-9][0-9]"
case "$CMDLINE" in *fromhd=/dev/*) DEVICES="$fromhd"; ;; esac
for i in $DEVICES
do
echo -n "${CRE}${BLUE}Looking for CDROM in: ${MAGENTA}$i${NORMAL} "
if mountit $i /cdrom "-o ro" >/dev/null 2>&1
then
if test -f /cdrom/$KNOPPIX_DIR/$KNOPPIX_NAME

To ztws01: Look at http://www.knoppix.net/wiki/USB_Based_FAQ, it explains the methods i used
to get as far as I have done. Unforunately i have to run some applications that require kernel 2.6.