You might try to do the mount operation manually. Which Knoppix version do you use?

When I got such an error message I opened a terminal and entered
Code:
sudo mkdir /media/myHdd
sudo mount /dev/sda1 /media/myHdd
Then I got an explanation what went wrong and what I had to do next.

Maybe you have to use another partition instead of /dev/sda1. You can get a list of the available partitions by executing
Code:
sudo fdisk -l