RAID Systems Technology . A simple replacement of one drive by a group of drives will not increase reliability since the life of the entire system will depend on any of this drives. This is why a certain level of redundancy is needed in the design of drive array to increase reliability of the entire storage system.
RAID is an assembly of disk drives, known as disk array, that operates as one storage unite.
In general, the drives could be any storage system with random data access, such as magnetic hard drives, optical storage, magnetic tapes, etc. Redundancy of data at a chosen level. Depending on the level of RAID that you are using, this method of data storage provides the data redundancy needed for a highly secure system, with additional benefit of faster retrieval of data through multiple channel access. If one or a few disk drives fail, they can be normally exchanged without interruption of normal system operation. Thus, disk arrays can ensure that no data is lost if one disk drive in the array fails.