How can a startup volume faithfully back up itself?

[dnanian]

If you're worried about files changing after you've backed them up... well, they're going to do that once you start back up from the drive, too. But, most definitely: if you want to capture the drive state exactly as it was when you hit "Copy Now", you can't run from it, because things could change. But those things are typically not troublesome -- you're the user, after all, and you know what you're changing that's significant.