Virtual Machine USB Boot is a portable tool which makes it easy to boot a virtual machine - VirtualBox or QEMU - from a USB key.
Setup is simple. We clicked "Add", entered a name for our project, chose the VirtualBox VM to be launched (these were automatically detected and presented in a list), and the USB drive to boot from. That's all you need to do, although there are other tweaks available - alternative load methods, run minimized/ full screen options, CPU priority choices - for anyone interested.
Every boot item you create is added to a list. Double-click one and it dismounts the USB key from your PC, adjusts the VM's settings to include it, then launches the VM for you.
This worked perfectly for us. We double-clicked our test entry, the VM appeared and booted from our USB key. The key wasn't then available to our host operating system - you can access it from one system, or the other, not both at the same time - but once we closed the VM, Virtual Machine USB Boot mounted the key and we were able to use it again.
Running Virtual Machine USB Boot doesn't affect your regular VirtualBox installation, either, because it restores the original VM settings when it's done.
There are a few VM configuration issues which might cause problems. Virtual Machine USB Boot tries to add your key to the first available port in the VirtualBox storage controller, for example, but this will fail if there's no port available. Fortunately pop-up alerts warn you of the issues, and if you do run into problems then they'll probably be simple to fix.