I have heard a good bit of "Drobo vs. WHS" and it should be recognized that Drobo is a USB attached drive. So in effect, Drobo is a compliment to your WHS and can attach just as any other external USB drive, and offer you redundancy without forcing the user to go through the hassle of choosing which folders or files to clone...

David Berlind mentions the ARS reivew - check out the ARS review at: http://arstechnica.com/reviews/hardware/storage-robot-at-your-service-a-review-of-the-drobo.ars