I have installed ubuntu on an external hard drive from linux using qemu directly and then booting my macbook (2013 model) on it. something like this should work if the external shows up as sdb. might be able to do something similar from os x with virtualbox.
kvm -m 1024 -drive file=/dev/sdb,if=virtio,cache=none -cdrom proxmox-3.4.iso -boot d