RAID (Redundant Array of Independent/Inexpensive Disks) technology is a great innovation in storage technology, allowing something that is fundamentally unreliable and slow—hard drives—to become faster and more robust. As with any technology, however, you should understand the tradeoffs and not blindly apply it to real or perceived problems. I had the opportunity recently to learn this first hand. The experience was such a "D'oh!" that I thought it worth passing on.
In concept, RAID 5 NASes provide a great way to get a large amount of storage while lowering the odds of data loss by disk crash. RAID 5 systems are more efficient than RAID 1 (mirrored) because you lose only the equivalent of one drive's capacity in an array of three or more drives. RAID 1 systems lose 100% of the capacity of the second drive.
However, as was made evident from my recent experience, RAID only provides insurance against drive failure. If some other part of your NAS system goes South, all of that wonderful RAID goodness won't make your data available.
What happened to me was the the controller board in my RAID 5 NAS went belly up. It could just as easily been the power supply, which I actually first thought was the culprit and wasted a week replacing. Or the main or power supply fans, whose failures should have caused a thermal shutdown.
The good news is that I use my NAS only for backup of my primary work machines. So the only thing I lost during the two weeks that the NAS was down was the comfort of knowing that my files were backed up every four hours.
But what I had been using the NAS as my primary file store? Or been burglarized? Or had a fire or unexpected flood? All of those wonderful RAID parity bits would have been useless because they were all part of the same box.
The fact is that most home, SOHO and smaller business users don't need the constant availability that RAID was designed for. While centralized databases supporting dozens of transactions per second do need to be able to stay up and running if a drive dies, less intensive uses can take a different approach to data security.