This can be achieved by employing redundancy in the form of error-correcting codes to tolerate disk failures. A redundant disk array can now retain data for much longer time than an unprotected single disk. Do not confuse between reliability and availability. Reliability is how well a system can work without any failures in its components. If there is a failure, the system was not reliable.
Availability is how well a system can work in times of a failure.
If a system is able to work even in the presence of a failure of one or more system components, the system is said to be available. Redundancy improves the availability of a system, but cannot improve the reliability. Reliability can only be increased by improving manufacturing technologies or using lesser individual components in a system. Every time there is a write operation, there is a change of data. This change also, has to be reflected in the disks storing redundant information.