I haven't actually done any testing with a USB key yet. I have a VMDK that 6.3.5 was installed on. It booted just fine, and I could access the GUI via the IP in the console. Obviously I couldnt set up an array or anything, but I could at least boot and access the GUI.
I upgraded that VMDK to 6.4.0, and it I get the error listed above.
I planned on doing some more testing with another USB key today, but for some reason, neither my 2nd or 3rd key are licensed. I own three unraid Pro licenses, so I need to get that worked out first.