Not only isn't it supported but Windows as it comes from Microsoft does not install or run from USB drives. So not only does Microsoft not support that but also Apple would have to alter Microsoft's OS (and more than one?)
What's wrong with just running Bootcamp? You can boot either OS.
Bob
I currently have an iMac with Windows XP on a Boot Camp Partition. I only have one internal drive (iMac, April 2008 model), so I can only have one Windows partition. I need Windows Vista for work but I still need Windows XP for most of my personal work. I do have an external USB drive; is there a way that I can "fool" the Windows installer into thinking that the external drive is really internal and therefore install & boot Windows from the USB drive? Otherwise, is there a solution that doesn't involve me wiping my Mac OS X startup disk?

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