A RAID (redundant array of independent disks) is a way to string individual hard drives together so that they behave like a single giant hard drive. These drives are usually linked together in a network-attached storage device or storage area network. The drives comprising a RAID can be linked together in many different ways. Each level has its own strengths and weaknesses.
People tend to rely on RAID arrays to both expand their data storage capacity and provide a cushion of support in case of drive failure. Some RAID levels, such as RAID-3 and RAID-4, have one drive act as a container for all of the special “parity” data which the RAID controller can use to reconstruct any missing data. Other levels, such as RAID-5 and RAID-6, distribute this parity information across all hard drives in the array.
Different RAID levels have different tolerances for drive failure—for example, RAID-0 will fail if one drive fails, RAID-1 and RAID-5 will fail if two or more drives fail, and RAID-6 will fail if three or more drives fail. As soon as a single drive in a RAID-5 or 6 array fails, it is prudent to replace it before any subsequent failure puts the safety of the data in jeopardy. The RAID controller will use its parity data to integrate the new drive into the array and continue to function normally. This is called “rebuilding” the RAID.