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>