Disk drives are the most commonly used secondary storage devices in computer systems. The way operating systems access these devices leads to a wide range of variability in access time. In this report we study the detailed temporal characteristics of disk drives. We describe a comprehensive set of experiments designed to build a model for the disk drive. Simulation is used to validate the model. This disk model will help design a device driver which can achieve a high degree of temporal determinacy. (Also cross-referenced as UMIACS-TR-97-14)