The Problem I have is how to install Ubuntu in a very tricky environment: My computer is a Macbook Pro (bought in 2013). My current OS is Mac OS and Windows 7 in bootcamp. Since my Mac Pro only has a 256 GB HDD, I would like to install Ubuntu on an external disk, so I went ahead and bought a USB3.0 Portable Hard Disk, which has 500GB space.
How should I proceed to install Ubuntu 14.04.1 LTS on an external HDD and boot it from there?
I would like to avoid to install it in a virtual machine.