Vista 64 and XP Dual Boot

[Guest Leadfootlady]

Maybe this question has been answered before, if it has, I apoligize. Can

you install a XP Pro dual system (Vista Ultimate 64 bit preinstalled by HP on

a SATA drive) onto an external USB Western Digital Hard Drive? Or is it

required to install on the internal sata drive with vista?

 

Thank you for your help.