Boot Windows from USB?

I have a perfect Windows installation, already with boot camp drivers, on an external USB drive. It's NTFS. I installed it by inserting the disk inside the Mac Pro's, using internal Sata connections. Then removed it, and now I want to boot it via USB.

Boot camp is not working, because my primary OSX installation is on a RAID array.

For some reason, rEFIt isn't working either.

What else can I try?