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>