hi I'm testing actual the same situation.
i have 2 ESX 5 Hosts.
while running a VM on Host A i remove the USB Stick.
now i'm still able to migrate the running VM from A to B.
But it's not possible to switch back the VM (from B to A again)
i even do not get a warning in Virtual Center if i remove the USB Stick.
I see only warnings in vmkernel.
anyone ever thougt about a monitoring for the USB stick i bootet from?
Robert