Windows, unlike most Linux distros, is not designed to run for an external hard drive, and 99.99999% of the time it will fail. (Microsoft debated adding such functionality, but decided it was not worth the effort.) There are many who claim to have done it successfully, but it requires modification of system files (without tutorial) and just the right hardware (which no one can specify in advance) to do so. Thus, the odds are overwhelmingly against it. Also, note that OSes are not automatically detected...the first step would be to install XP while the HD is internal, then move it externally and reinsert the Vista drive, using VistaBootPRO to add the appropriate reference to XP, but due to the above it would still fail.
I like to run XP on a Vista machine (Acer 5720).
There are many info on installing and running XP on a USB HDD....
My question is:
If I put a new, blank HDD in my laptop and install an XP on it, can it boot later as an USB HDD after I have put back the original Vista ?
Of course, instead of the multiboot option I set the boot device in the BIOS....
So ... Would that work?
Does XP install itself "remembering" that it sits on an IDE HDD or just happy to find the MBR on the boot device ?