Those groups only allow your user to auto-mount removable devices through HAL/DBUS, from inside an appropriately equipped desktop environment (KDE or XFCE); it has nothing to do with permanent devices like internal HDDs.
For a normal user to mount and use those, they would need to be manually added to /etc/fstab.
For example, a line to mount a NTFS drive might look something like this:
Code:
/dev/sda1 /mnt/windows ntfs-3g defaults 0 0