The main objective of the tertiary storage level is to provide huge storage capacity at low cost. Several types of storage devices are available to be used at the tertiary storage level in Hierarchical Storage Systems (HSS). They include: • Magnetic tapes • Optical disks • Optical tapes These storage devices are composed of fixed storage drives and removable media units. The storage drives are fixed to the computer system. The removable media unit can be removed from the drives so that the storage capacity can be expanded with more media units. When data on a media are accessed, the media unit is accessed from their normal location. One of the storage drives on the computer system is chosen. If there is a media unit in the storage drive, the old media unit is unloaded and ejected. The new media unit is then loaded to the drive. Each type of storage drive may handle the storage drives and media units differently. The magnetic tapes are described below in the next section. Then, the optical tapes are presented. Afterwards, the optical disks are briefly described before this chapter is summarized.