Hi knp,

I would boot from a Knoppix DVD and open a root terminal. Then I would connect the USB HDD and read the last kernel messages
Code:
dmesg |tail -n 40
If there were no errors I would check the filesystem on the USB HDD for errors and try to fix them if any occured.

Then I would try out the mount command and watch if there were any errors.

If there are any issues tell us precisely what you have done.