Additional info:
When I use the terminal and mount the partitions with:
knoppix@Microknoppix:~$ su
root@Microknoppix:/home/knoppix# mount /media/sda7
root@Microknoppix:/home/knoppix# mount /media/sdb5
root@Microknoppix:/home/knoppix#
then I have access, although, if I want to unmount from the File Manager (e.g. Dolphin) I get the same message - Not authorized to perform operation.