We would like to thank Arvind Krishnamurthy for many interesting discussions on the proofs of several analytical models, Marvin Solomn for discovering the simplest proof of the single track model, Daniel Stodolsky and Chris Malakapalli for helping us understand the working of Quantum and Seagate disks, Jeanna Neefe Matthews for asking the question of how hole-plugging could be efficiently implemented without hardware support for self-describing disk sectors, Geoff Voelker and John Wilkes for comments on early drafts and presentations, and Margo Seltzer for a large number of excellent suggestions during the shepherding process.