After updating KDE to 4.9.5 my system stopped auto mounting external USB devices. The fix was to change udisk policy in /usr/share/polkit-1/actions/org.freedesktop.udisks.policy <action id=”org.freedesktop.udisks.filesystem-mount”> <description>Mount a device</description> <message>Authentication is required to mount the device</message> <defaults> <allow_any>no</allow_any> <allow_inactive>yes</allow_inactive> <allow_active>yes</allow_active> </defaults> </action>