> Perhaps I'm showing my ignorance of USB, but how about another USB
> capable motherboard as the test device at the other end?  That way you
> would get to instrument the data stream at each end.
Sorry, no go.  USB is highly asymmetrical.  The host is the
master of the bus and is in complete control.  You cannot connect
two host, not even electrically since the plugs are different
and the two ends of a cable.  I saw someone making a USB device
that had two upstream ports (thus allowing two hosts) so that you
could make two hosts talk, but I don't think it was cheap.

        -- Lennart