Great news from Ben, aka as Virtual PC Guy. Microsoft made available tools and procedures that enable you to boot Hyper-V R2 Server from USB device.
Be aware of limitations!
For this reason Paul’s tool defaults to creating a 6000MB virtual hard disk. You can change the size of the virtual hard disk using the File menu.
This is not supported (if you read the whole TechNet article you will see that this is only supported if you are using a USB device that is hard-wired to the system).
The virtual network switches will get disconnected whenever you move to a new system.
Full article from Virtual Guy.