Backup vs. fault-tolerant storage
People sometimes confuse backup with fault-tolerant storage. A backup is a separate copy of a file, written to a different device. It should be able to survive even when the primary fails. Some storage, such as RAID devices, looks like backup at first blush, but they are really better described as fault-tolerant storage. Fault-tolerant storage has some internal protections against failure, but it does not provide enough protection to be considered a backup.
A drive spanning device such as a RAID 5 or Drobo can preserve the data in the event a single drive malfunctions, but the device itself is still a single unit, and has nearly all the vulnerabilities that a single copy of the data has.
Don't confuse fault-tolerance with true backup.
