You will need to reboot and enter BIOS [ start up] to change the boot order .
That tells the hardware which drive to look for the OS on .
EDIT : it might be possible to have one drive in a USB housing , then set the computer to boot from USB first , then hard drive . That way it will boot from the external if its plugged in and from the internal hdd if its not . Maybe someone else can tell us if that would work .