Automated tape libraries. Magnetic disks Magnetic disks have dominated secondary storage since 1965. Magnetic disks are used for two main purposes. They provide the lower level of the virtual memory system. They provide long-term, nonvolatile storage for files.
Disks are usually the highest level of the storage pyramid that is non-volatile.
Platters are 1 - 8 inches wide. Surface Each platter has two surfaces, both of which may be used for recording. Track A surface is divided into tracks, which are concentric circles containing data. The set of tracks at corresponding locations on all of the surfaces is called a cylinder. Sector A track is divided into sectors each of which holds a fixed amount of data, usually 512 - 4096 bytes.