RAID level 2
  Error protection provided by ECC (error correcting code)
  - Hamming code 

  Data distributed at bit level to work with Hamming.

  Allows data to be corrected after fetching (on the fly).

  Requires large number of disks
    Hamming e.g 4 bits data + 3 bits error. (7 drives minimum).
    Improved data to check bit ratio as size increases.
    10 + 4, 32 + 7

  Disks must be synchronized.

  Only one file may be accessed at a time, but very high throughput.

  Economical only if using a very large number of disks.

  Provides both disk failure protection and corrupted read/write.

  Requires customized controllers. 

  Because most other RAIDS are much cheaper, not implemented.