Well - the right way to specify the access to the usb-drive is /etc/fstab and nothing else.
You do it once, and - voila.

I guess /etc/fstab is older than X11, KDE, and usb-drives.
And linux was multiuser-system ever, where it's possible and neccessary, to restrict rights to write to files on a per-user level.

And the knoppix-team decided to make knoppix as secure as possible, not as userfriendly as possible, to prevent newbies from damaging something.

If you want to leave newbie-state, read man fstab and man mount.