There are two types of mass storage devices: magnetic disks and removable media. For storage system based on hard disks, one can use disk array such as RAID (Redundant Array of Inexpensive Disks) to increase data redundancy for reliability. One of the advantages of such a system is that all the data can be available on-line all the time. However, additional cost of secondary backup on tapes may need to be considered. And the initial setup to create data redundancy may be expensive.
The other alternative is the use of removable media. For most of the high density removable media, devices that uses robotic mechanisms are available to build mass storage systems. These are sometimes called auto-changers or juke boxes, or in the case of tape media, (tape) libraries. With such devices one can build a semi-online data archiving system with large volume capacity.