Too little information was provided. You should at least provide the command you use to mount the harddisk, and the corresponding dmesg log. I suspect it is likely a matter of /etc/fstab corrupted resulting in errors in commands such as 'mount something' ( ie a mount commands which takes it's parameters from /etc/fstab ). Ask in debian knoppix list is not a bad choice but this certainly does not demand a direct attention from anybody in particular.