I second this observation. I suspect that Mac OS X keeps track of files that were changed since the last TM backup (regardless of volume) and only updates those files. The fact that the first backup after you switch drive is as fast as a backup on the existing drive is somewhat indicative of this behavior. I stopped using multiple drives for TM long ago and switched to a TM+SuperDuper redundancy strategy. It takes a very long time to mirror a TM backup drive (even with Smart Update), but I only do it once every two weeks.

The fact that the first backup after you switch drive is as fast as a backup on the existing drive is somewhat indicative of this behavior.

That's totally not what I am observing, and not what I've been validating using tools like Time Machine Buddy and TimeTracker.

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