Install Windows 7 on external drive and boot it from a Mac
It is possible, but it's not straight forward. This detailed step by step guide (which I did) contains all you need to know to install and run Windows 7 or 8 from an external drive (USB3 or thunderbolt): http://bleeptobleep.blogspot.fr/2013/02/mac-install-windows-7-or-8-on-external.html

To sum up, this guide will help you:


Get the Windows installation pack from a DVD or ISO file
Partition and format your external drive to make it bootable
Deploy Windows on your external drive
Boot on your external drive
Install Windows
Download and install the bootcamp drivers (without even using the bootcamp assistant)