Here is what I ran into booting on two disks. After successfully installing XP and Ubuntu on separate disks, (XP should be first), the ubuntu grub gives you a choice of which OS to open. This is fine........untill you want to or have to replace ubuntu. When you take it out the bios can no longer find Windows. The bios is still controlled by Linux. The only safe way to use them that I have found is to change the boot in the bios. Surely someone can come up with a safe and easier way to boot either without having to change the bios or use the grub.