RAID 2

RAID 2 is hardly used anymore. What happens in a RAID 2 configuration? According to ACNC.com: 

Each bit of data word is written to a data disk drive. Each data word has its Hamming Code ECC word recorded on the ECC disks. On Read, the ECC code verifies correct data or corrects single disk errors Interleaving by bit or block.

Data is protected by Hamming code (a mathematical error correction code) and RAID 2 uses extra drives to detect 2-bit errors and correct 1-bit errors on the fly. 

We believe that you need a minimum of six disks. This includes five data disks and a minimum of one ECC disk. 

RAID 2's advantages are that it is fast and the controller is relatively simple but the downsides that make it a solution not used much anymore and not a very popular solution even in the past are that it is inefficient and expensive for what it does. Hardware integrated ECC in modern drives makes it even less appealing.