you hvae to unmount the card BEFORE removing it.
then you mount the new card to be able to see the files in it...
a sync (just type sync in a conole and hit enter) before unmounting ant then removing it might help, should not be nessesary though.
oh, one more thing, close anything you have weived files on the card with. or atleast go to the /home/knoppix directory, otherwise umount will just say thet disk is bussy.