I'm using a macbook and am trying to use one usb-drive to install ubuntu onto another usb-drive and boot from that other usb-drive.
I created an installationdisk for ubuntu 14.04, and then installed it onto another usb-disk, following theese instructoins: https://www.youtube.com/watch?v=QdQ520dmg5g
I unplugged the installationdisk, keeping the installed disk plugged in. While holding the key "alt" i booted up my macbook pro. Holding alt should show all bootable drives on a mac, but I only get my osx system disk and my recovery disk. The usb-stick with ubuntu doesn't show up. The thumbdrive that I used as a installationdisk worked fine .