Wonder what would happen with a third drive? LOL...
You are so smart for doing that. I keep two laptops up-to-date so I'll have one if the other goes down. I don't have a desktop anymore. I have to take mine with me all the time, so I never used my desktop when I had it. I gave it to my folks, who have since replaced it with a new one. I think I'll get them a spare HD, though, and back up their other one on it plus have it ready "in case". I think that's just a grand idea.
With the prices of the HDs going down, you could do just a regular backup of your main drive, & store it on a HD, rather than backing it up to cds or dvds. Saves time, too!
Actually, I DO have a third drive with XP on it, with some programs that I use for DVD movies.