I just had a bit of an issue on my Slackware laptop where my USB flash drive wouldn’t mount as a user. In KDE, plugging in the USB drive would have the notification pop-up come up, but it wouldn’t let me mount the device. When I checked out /var/log/messages, I ended up seeing this particular error:
Jun 2 20:55:45 lappy dbus-daemon: [system] Rejected send message, 1 matched rules; type="method_call", sender=":1.11" (uid=1000 pid=5137 comm="kdeinit4: kded4 [kdeinit]) interface="org.freedesktop.Hal.Device.Volume" member="Mount" error name="(unset)" requested_reply=0 destination="org.freedesktop.Hal" (uid=0 pid=1886 comm="/usr/sbin/hald))
It looked liked a permissions issue. After checking /etc/dbus-1/system.d/hal.conf I could see that there was permissions set in place for the plugdev group to mount USB drives.
The fix was really simple. I just logged in as root and ran usermod -a -G plugdev *user*