Hi :)
It sounds like you are trying to use a usb-stick to boot-up a VM?? I didn't think this was even possible?!
I have had usb-sticks booting up desktop machines. Usually i do a full install of the distro onto the usb-stick and Grub finds the various OSes on the particular machine i happen to be using at the time. Those extra menu items don't work when i'm on another machine (of course).
I thought VirtualBox didn't like letting people use usb-sticks until you pay for the "professional" version (or whatever they call it)? Also i wouldn't have thought that VM drives get mounted until after the OS they are in has been booted into?
Hopefully that is all just down to my almost complete lack of knowledge about all this.
Regards from
Tom :)