Windows – How to boot Windows XP on a Mac from an external drive

I have an external drive that uses the GUID partition scheme, and has the partition info mirrored to an MBR as Macs are won't to do. The drive does FireWire and USB, and Ideally, I'd like any solution presented here to work with both buses.

I have HFS+ and NTFS partitions on it. The NTFS are among the first four as MBR needs them to.

I did this partitioning via Disk Utility, and then used Winclone to copy my Boot Camp partition to the NTFS partition on the external drive. The cloning went fine, the files are there, but the Windows partition does not appear to be bootable.

I tried using rEFIt, by keeping it on another partition on the external drive and booting from it, but that was to no avail. I still got a "No bootable device…" message.

The only way that I was able to boot from the external drive was by keeping a minimal Windows installation on a partition in the internal drive. If you scour the web you'll find a way to do this using only a minimal partition (60mb or so) with only the very necessary files there. But I really wanted the external drive to be reusable among many Macs.

update: it has since come to my attention that Windows XP does not boot out of the box from external devices, but that it can be hacked to do so. My googling and reading shows people doing new installs to achieve so. And it's all very clumsy and there doesn't seem to be a definitive way to do this[1]. Can I hack an existing Windows XP install to be USB and FireWire-bootable? Also, most solutions focus on USB thumb flash drives, and I'm talking about real spinning harddisks.

So, most importantly, is it at all possible to boot Windows XP on a Mac from external drives?

I have spare drives aplenty to play with, and so I have been doing, but so far I've mostly managed to mess up my internal disk partitioning scheme in a very interesting, peculiar and irrelevant way. (SuperDuper to rescue). So far, all FAIL.

[1]: The Mac of Windows hacking, so to speak.