RAID is not a substitute for a proper backup.
RAID reliability is provided by redundancy. In theory, the probability of a simultaneous two-disk failure is the square of probability of a single disk failure, but that formula only works given that the failures are independent.
Actually, the drive failures are not independent because there are many factors in common for all drives in the array.
These factors include:
- Temperature. If a drive is damaged as a result of the overheating, most likely that rest of the drives overheat as well.
- Power. If a power supply burns out (or lightning strikes the power line) all the drives would fail immediately.
- Logical connection. If you have RAID 1 and you have accidentally deleted some files, both copies would be deleted simultaneously.
- Controller. If a RAID controller burns out, a disk array would go offline completely. In a lucky case, it is possible to attach the drives to a similar controller and it would recognize the array, but it is not always that smooth. Sometimes, a RAID recovery software might be needed.
- Cables. If several drives are connected to the same cable (as it was earlier with IDE and SCSI) and the cable snaps, all the drives connected with this cable would be gone.
There are multiple reasons why the redundant array may fail instantly, and the proper backup is still required to provide a secure data storage.