ok I could manage to correct the bug.
it is a minirt.gz bug, so you have to mount as cloop your cdrom, then go to /boot/ and get minirt.gz, copy it somewhere and gunzip it. then, in the line that adds startable devices, the line containing

/dev/ub?[0-9]

is not good anymore, because on new kernels (dont know which version, maybe somewhere on >= 2.6.11 ) usb devices are named now /dev/ubx

the problem with that line you see above, is it implies that ubx device has partitions ! BUT ! and usb stick doesnt have (maybe it does), but certainly not cdrom usb devices ! so you MUST add

/dev/ub?

before the /dev/ub?[0-9] in order to take into account that if you boot with a usb cdrom drive, it will get a name like /dev/uba and not /dev/uba1 !

That's why Knoppix was bugging !

so, next steps from where we were are: mount minirt (uncompressed miniroot) somewhere, go there and find the linuxrc startup script, then add what i previously said, then unmount, gzip, then rebuild the iso image and you are ready to go !

have nice day