RAID 
redundant array of independent disks 
A hard drive subsystem that provides convenient, low-cost, and highly reliable storage by saving data on more than one disk simultaneously. RAID tends to be used on servers rather than on personal computer systems. There are many different types of RAID. A RAID-0 consists of two drives that store different parts of the same data sets (known as interleaving the data); this improves performance, because when one set of data is called for, it can be accessed from two drives at the same time, but it does not safeguard against failure. A RAID-1 array does, however, by consisting of two drives that store identical information. If one drive goes down, the other continues to work, resulting in no downtime for users. To save disk space, RAID-3, -4, and -5 stripe, or distribute, data and parity information across multiple drives (RAID-3 and -4 store all parity data on a single drive). If a single disk fails, the parity information can be used to rebuild the lost data.