Are you trying to install windows on an External hard drive because you are going to connect it to another computer.... That wont work on the newer version of windows cause, I trued it before. It will give you an error message cause the devices ex. video card, sound card. Will be diffrent then what it has stored. Id you just want to install 2 diffrent OS, I recomend you have an Old OS install first then install the new OS as new clean install instead of update. That really would work better. I Don't think Windows will work on external hard drives because 1. You always have to transfer infomation back and forth and a USB firewire can only go so fast. 2nd. If your wire becomes loose or falls off for some reason it will hurt the computer