—rigid or hard magnetic disks; —parallel disk arrays; —optical disks, which have several variants: read-only [such as CD-ROM and digital video disk (DVD)], write-once (WORM), and rewritable (magnetooptical or phase change); —magnetic tape, such as 4mm (DAT), 8mm, and digital linear tape (DLT); —autochangers, which combine storage devices with racks of tape cartridges or optical disk platters that can be moved to and from the drives by robotics. —Complete storage systems also include software to manage and orchestrate the storage components. Such software includes management and configuration utilities for storage devices, logical volume managers that tie multiple physical devices together as one, and file systems to arrange layout of data on storage devices. —Storage systems are a vital and growing market. Their primary components are magnetic and optical disk drives, magnetic tapes, and largecapacity robotic assemblies of drives and cartridges. Storage hardware sales in 1995 topped $40 billion, including more than 60,000 terabytes of hard disk storage. In recent years, the amount of storage sold has been almost doubling each year; in the near future it is expected to sustain an annual growth of about 60%. This enormous growth rate has been accompanied by a 50% per year decrease in the cost per byte of storage. These growth trends exceed those of the personal computer market. —The focus of this article is on secondary storage systems and primarily on magnetic disk drives, since these are