For your reference here is your manual. On page 22 under your Boot section in BIOS.
Boot Menu: • Disabled • Enabled [Enabled] This field is not active unless the Supervisor’s Password has been set. When disabled, access to the Boot Menu with the [F12] key is disabled. When enabled, the Boot Menu is enabled and the [F12] key will allow you to display it.
Setting a Supervisor Password (page 17) will allow you to tap F12 at boot to select your boot device. That may help. Keep the password simple. It may already be enabled.
When you say the computer refused to boot from an external drive, are you referring to a bootable USB flash drive. I am not sure this old computer could boot from anything other than a CD I doubt it could boot from a USB external hard drive or USB flash drive. Looking at page 23 the only boot options are Floppy disk, CD, or hard drive. Hard drive being an internal, not external.
Edited by JohnC_21, 16 June 2018 - 06:49 PM.