How old is the motherboard and how is the drive connected to the motherboard? Some boards will not support booting from external drives
What operating system is on the external drive? and are you booting the drive up on the same computer the OS was originally installed on? If you installed the OS on another computer first and are wanting to use the install on another computer then it probably won't work.
Try a different key on bootup to get to the BIOS/bootmanager. check the manual for the correct key