Tente isso. Apenas pressione Ctrl + Alt + T no seu teclado para abrir o Terminal. Quando se abre, execute o (s) comando (s) abaixo:
sudo gedit /usr/share/polkit-1/actions/org.freedesktop.udisks2.policy
Quando se abre, procure
<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>
E mude para
<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>