Virtualbox can modified to boot from usb or external hdd usb with OS system in it.
Here is the command
VBoxManage internalcommands createrawvmdk -filename usb.vmdk -rawdisk /dev/sdbx
We can replace /dev/sdbx according to the mounting point of the external usb.
Next, create a new machine in virtualbox and put the vmdk we create into the hdd (sata) option. Don’t unplug the flash disk in this step.
Next, click run to see if the external flash is working.