Perhaps it has been assigned as something else. Is this message reported on the desktop or are you mounting manually?

Right after you plug it in, open a terminal session and run the command: tail /var/log/messages ; to read the end of your message log. There should be information about what the kernel has found at a usb bus, the driver installed and the device descriptor assigned.

This should all happen automagically. Try booting with the device attached to the system and if you continue to have issues, please post back with the knoppix version you are using and your exact actions.