If its like Vmware, then you have two options, one doesn't work, the other should.
If you select connect the USB for that external drive, then it would be like running windows natively and hooking up a HFS+ drive to it, i.e., Windows by default cannot read HFS+ drives. I suspect you may have tried going this route.
The other option is to select a share location, for Vmware Fusion, I go to Virtual Machine ->Sharing->Sharing Settings and from there I define a location whether internal or external and its then basically a network drive, and the external drive's format doesn't matter. Does that make sense?
tl;dr Try setting a share point on the external drive through VB.