I don't think that I had noted the point that the device must be
un-mounted on the host OS. Perhaps this will solve my problem.

But actually, as I said, I am not particularly concerned about this problem
and am very happy as I am and have no great need to use the USB under the
guest OS. I will merely trying to reassure the original poster that it was
not a problem unique to him (or her?)

Incidentally I am running VirtualBox twice - Windows XP under an Ubuntu host
and Ubuntu under a XP host. Both work great.