Hey, I am also facing this problem since last week,
Actually my condition is , i am trying to install(not live) kali linux amd 64_1.0.9a on a pendrive(32gb) through the virtual box , so i can use the pendrive as my hard drive to booot into kali , as i mention ,i am facing this problem since last week before it i was able to boot it from usb and every this was right ,i format it for some resons and know trying again to do the same but now i am unable to boot it from usb ,i am also attaching a attachment of my error when i try to boot it from my usb after installation of kali through virtual box(installed completly without error) , any suggestion and reply will be highly appericiated fer.JPG