Running the VM on an external, while very much possible, will result in a greater performance hit on top of what you might experience anyway because of virtualization. In other words, VMs will be slower that running on native hardware and running it off of an external is only going to exacerbate the problem. I've done it before (booted a VM off of an external) and it's manageable but not useful for anything beyond simple tasks.
What do you plan to do with your Windows install? If your needs are not dependent on resource intensive or 3D application use, a VM might suit your needs. Also, happy birthday.