1. Open a terminal and paste this:
sudo gedit /usr/share/polkit-1/actions/org.freedesktop.devicekit.disks.policy
2. In the file you opened with Gedit above, search for this:
<action id="org.freedesktop.devicekit.disks.filesystem-mount-system-internal">
<allow_active>auth_admin_keep</allow_active>
which needs to be changed to:
<allow_active>yes</allow_active>
That's it.