Install an OS off of an external hard drive?

I have decided to install a different type of linux on a spare computer and my DVD+R CD’s are too small to cram the .ISO file onto, but i have an external hard drive that keeps some of my stuff backed up. So instead of having to go out and spend money on new CD’s for one thing is there a way that i can install the os off the external hard drive i have?
I have tried to just copy the .iso file onto the external/plug it in/switch the boot order and restart, but that leaves me looking at a black screen with a flashing white bar. Is there something I need to do to the file before attempting to boot?