I am not familar with Mandriva, but in FC6, to boot from a usb drive, the following modules need to installed during boot (before attempting to mount the real root filesystem): scsi_mod, sd_mod and usb-storage.
These modules may be built in the kernel on Mandrivia, but if are not, they need to loaded from the initrd during boot. You will also need a 'sleep 8' right after the usb-stoarge module is loaded to give the usb subsys time to find the usb disk, before trying to mount it.
Like I say, this works in FC6
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