No boot on external usb cdrom:can't find knoppix file system

I have a laptop Dell400 with external CDROM connected on USB.

The boot start until scanning USB/firewire
Ater that, it stops : can't find knoppix file system
and come to a very limited shell.

In the past, the boot was ok with knoppix 3.4 and 2.4 kernel.
Since then, it is impossible to boot from the external USB CDROM.
Same problem with derivative of knoppix (Kaella, ..)

I tried all options : nodma, nousb ..
I even tried bootfrom=/dev/sda1
Nothing

Thanks for somebody to help or correct this bug.
JC