Install Windows XP to USB Hard Drive and Run from It

I wanted to know if there was a way (that worked!) to both Install and Run Windows XP Pro from a USB 2.0 connected Hard Drive. My system does allow to boot from external usb hard drive, but when I tried to install Windows XP to the USB HDD, it didn't allow me to.

My ultimate goal is to be able to move only the hard drives (w/ windows installed) to other identical systems, in times of maintenance, etc.

The Old New Thing: Why can't I install Windows on my USB drive? (April 15, 2004)

[...] Another reason not mentioned in this paper is that during any hot-plug operation, the USB bus is completely reinitialized. Windows really doesn't like it when it loses access to its boot device. [/...]

The whitepaper mentioned there is Recommendations for Booting Windows from USB Storage Devices.

Possibly related: A Deep Dive into USB Boot (MSDN)

The trick is actually on the BIOS to properly emulate a Int 13h device for the Master Boot Record. I've found the ASUS motherboard do this properly.

One "trick" I've found is to install it to a normal SATA drive then migrate the actually HDD into the USB case. This generally works as the USB drivers have already been installed into the "kernel space" during boot, and the system will continue.