I have 3 external USB hard drives accessable and used in both Kubuntu
and XP on VirtualBox simultaneously. The setup is not very intuitive.

The USB extensions and the Guest additions must be installed and the
USB devices must be shared by (K)Ubuntu on the network. They must be
mounted using the "shared drives" section of the VirtualBox main
screen. The drives mounted in Kubuntu are then available for XP use.
The "network" drives are actually on the same box, but can be accessed
on the virtual machine.

Sorry to be kind of vague but I'm not at the computer with VirtualBox
in use now, but it can be done. Rather awkward but possible. Others
may be able to explain it better... and I invite them to do so.