Stop asking for authentication to mount USB stick
Solution 1:
Try this. Just press Ctrl+Alt+T on your keyboard to open Terminal. When it opens, run the command(s) below:
sudo gedit /usr/share/polkit-1/actions/org.freedesktop.udisks2.policy
When it opens look for
<action id="org.freedesktop.udisks2.filesystem-unmount-others">
<defaults>
<allow_any>auth_admin</allow_any>
<allow_inactive>auth_admin</allow_inactive>
<allow_active>auth_admin_keep</allow_active>
</defaults>
And change it to
<action id="org.freedesktop.udisks2.filesystem-unmount-others">
<defaults>
<allow_any>yes</allow_any>
<allow_inactive>yes</allow_inactive>
<allow_active>yes</allow_active>
</defaults>
Solution 2:
I had the same problem today; "all of a sudden" my 14.04 wanted my password for "everything"; including (un)mount of USB sticks.
I figured that I installed openssh-server in the morning. After removing it, everything is back to normal; I can insert a USB stick; and it is mounted without asking for a password. Strange. Just tried to repro; installed openssh-server again; and no ... no such effect this time.
Solution 3:
I had the same problem with mounting on one of my systems. I tried using @Mitch approach above which didn't work for me (Although it may work great if you have the unmounting issue) . I took a similar approach to begin with and
sudo gedit /usr/share/polkit-1/actions/org.freedesktop.udisks2.policy
but changed this section
<defaults>
<allow_any>auth_admin</allow_any>
<allow_inactive>auth_admin</allow_inactive>
<allow_active>yes</allow_active>
</defaults>
of
<action id="org.freedesktop.udisks2.filesystem-mount">
to
<defaults>
<allow_any>no</allow_any>
<allow_inactive>no</allow_inactive>
<allow_active>yes</allow_active>
</defaults>
matching content found in the old
/usr/share/polkit-1/actions/org.freedesktop.udisks.policy
Sources: @Mitch answer above
`/usr/share/polkit-1/actions/org.freedesktop.udisks.policy`
https://forum.kde.org/viewtopic.php?f=22&t=112092
Testing