Not able to boot external hard drive?

Hi, I have linux installed on an external hard drive. I am trying to boot it from my alienware 14 windows 8 A04 bios. Yet when I select legacy mode and select my usb device to boot from, all that happens is I get stuck in a boot with a white blinking cursor at the top. I know that the hard drive is bootable because I tested it out on windows 7 computers and it booted up just fine when I selected the device in the BIOS. Anyone know how I could get this working? I am new to this community and looking forward to meeting everyone! All help is appreciated!