Cita:BootCamp user Info/Tips on Booting Win XP from USB Drive
(in reply to a past posts/questions on using WinXP on an external drive from Intel CPU Mac owners)

"Mike... Found this for your readers:

Hello, I cannot find a working solution for this posted so I'll post the solution I came up with (well, cobbled together from other people's hard work). I am using a MacBook Pro and an external G-Tech Q drive that supports USB 2.0. While XP does not officially support booting from a USB drive there is a way around it.

This solution requires access to a PC and is a bit time-consuming.

1. Use BootCamp to create a 5GB XP partition.
2. Using a PC and your Windows XP SP2 CD, go through the following process to create a USB enabled XP SP2 installation CD:
http://www.ngine.de/index.jsp?pageid=4176
3. Plug your USB 2.0 external drive into your mac.
4. Put your new XP SP2 CD into your MAC and boot holding down the "C" key.
5. XP installation should begin, when you get to the drive choice, be sure to choose your USB drive. You may have to pre-format this drive to NTFS, mine was already formatted.
6. You will get a screen warning you that drive C needs to be formatted. This is OK because XP needs to write some temporary data to the startup (internal) disk. It will format the 5GB XP partition you created in step 1.
7. Follow the installation prompts and when your Mac reboots be sure to hold down the Option key. You will get a choice to boot to either your OSX installation, XP or the XP CD. Choose the XP hard drive.
8. The normal XP installation will continue. You will get some warnings about unsigned divers. This is fine and is occurring because you tampered with the XP installation files in step 2. NOTE: My installation hung once here. I just reboot and tried from step 7 again and it worked fine.
9. Once XP comes up install your BootCamp device drivers.

Good luck! I just now finished this so I have some testing to do. A very big thank you to emmanual from ngine.
-William R."

Fuente: http://www.xlr8yourmac.com/archives/apr06/041706.html#S20998