Yes, go to BIOS setup by presing F2 repeatedly as soon as you turn on your system, inside check if the MoBo detects ur external HDD, if it does, go to the boot section(Every Motherboard is different so i can't tell you exactly how to get there), set harddrive as first to boot, then set also set Harddrive priority, save config to CMOS, and it will restart, then it sohould 1st boot to that external HDD.
Note: I assume that you have an OS on that drive, thats why you want to boot from it, right?
Hope it helps
post your outcome