[Baron Munchausen]

He wasn't asking about multi-boot installation. He was asking about attaching two drives with two independently installed copies of the same OS to one computer. (XP 'Home' and 'Pro' are essentially the same OS.) I repeat, this is reputed to be dangerous. Directions for making backups by 'cloning' from one HD to another are very explicit not to try to boot the computer from the drives while drives with both copies are attached, but to only boot from the cloning utility (a CD, that is). Unless you have an elaborate boot manager like GRUB which can hide one of the copies from the other. Two different installations may not be exactly like a cloned backup and an original, but it sure isn't the same as a multi-boot installation where Windows has had a chance to sort itself out, either.