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Recovering a Wrecked RAID
Dr. Eggman writes "Tom's Hardware recently posted an article specifying how the professionals at Kroll Ontrack recover data from a RAID array that has suffered a hard drive failure, allowing for recovery of even RAID 5 arrays suffering two failures. The article is quick to warn this is costly, however, and points out the different types of hard drive failures that occur, only some of which are repairable. Ultimately the article concludes that consistent backups and other good practices are the best solution. Still, it provides an interesting look into the world of data after death."
That's true, but the most common cause of data loss on a RAID system that I've seen is when a disk fails, and people leave it there for days or even weeks without bothering to replace it.
When a disk fails in a RAID, it needs to be replaced IMMEDIATELY. A RAID system with a failed disk is a disaster waiting to happen. I've been in smaller shops that don't even have spare disks around. When a disk failed, they would order a disk at that point and have it shipped.
You should always have plenty of spare disks around, and you should replace disks as soon as they fail. A double disk failure is rare, but the longer you put off replacing a failed disk, the more likely it becomes.