I install with bootcamp to a partition on the internal drive. after install and a few reboots to makesure everything is working I clone the Windows partition to my external disk with winclone. and delete the internal install of windows. I done this on 3 machines and never had a issue. I also use winclone to back up my windows install in case I screw something up while using windows. To me winclone is necessary if you use windows on a mac.