guys watch out the script it uses is located at http://eazyhax.com/pitime/expand_to_external_drive.sh and adds avoid_warnings=2 to /boot/config.txt so remove it afterwards or you won't get notified anymore about undervoltage and high temprature warnings on your pi system. I only found out today.

I don't know about a bootable drive, but this guide might be close to what you want: https://github.com/retropie/retropie-setup/wiki/RetroPie-Ubuntu-16.04-LTS-x86-Flavor