HOW TO: Install Windows to a Macintosh USB drive (without Bootcamp)
OSX can do everything I want from day to day, but occasionally I want to play some San Andreas. Bootcamp and Windows should 
have had me playing Windows games on the Mac. Sadly, Bootcamp refused to partition my internal disk so I was stuck with no Windows and no San Andreas. A ray of hope was the idea of installing Windows onto a blank external disk. Windows is not supposed to run from an external disk but it has been done before. The brave souls who made this work in the past still needed to re-partition their disk with Bootcamp though, something that wasn’t an option for me.
It turns out that it is possible to install Windows to a USB disk without using Bootcamp at all. Here’s the recipe: Install Windows to a Macintosh USB drive (without Bootcamp). « Tube Shards