Can anyone diagnose this thunar-volman error please?

[Chris Green]

I am still trying to work out why *one* of my xubuntu 11.10 systems
doesn't automount USB and CD drives.

The most obvious difference between working systems (of which I have
two) and the non-working system is an extra error message from
thunar-volman.  E.g.

On the working systems:-

    chris at test:~$ thunar
    thunar-volman: Unsupported USB device type.
    thunar-volman: Unsupported USB device type.
    thunar-volman: Unknown block device type.

    ... and the USB drive is automounted


On the non-working system:-
    
    chris$ thunar
    thunar-volman: Unsupported USB device type.
    thunar-volman: Unsupported USB device type.
    thunar-volman: Unknown block device type.
    thunar-volman: Could not detect the volume corresponding to the device.


So can anyone suggest why I'm getting that "Could not detect the volume
corresponding to the device." on the non-working system and, more to the
point, how can I fix it.

It's quite possible that I have misconfigured or uninstalled something
necessary.  I have checked that all the obvious things like gvfs and so
on are the same on both systems.

-- 
Chris Green