Redundancy is a key feature of HA storage, as it allows data to be kept in more than one place and eliminates a single points of failure (SPOF). HA storage systems are often used in virtual server environments as a security feature to protect critical virtual machines.
HA storage arrays require at least two controllers in case one is lost. For disaster recovery, HA storage requires a secondary storage system to hold critical data and applications the business needs if its primary storage system goes offline. This is often done by replicating data to another storage system at another site.
See also: high-availability, five nines