RAID (of any kind) isn't a backup because it doesn't store previous versions of files. For example, if your RAID 1, 5, 6, 10, etc. array is attached as a Window

RAID (of any kind) isn't a backup because it doesn't store previous versions of files. For example, if your RAID 1, 5, 6, 10, etc. array is attached as a Windows shared network drive -- which is not uncommon -- then CryptoLocker can find it, silently encrypt every single .doc, .pdf, .jpg, etc. file on it, and you're still boned because you don't have a copy of those files before they were changed. Less dramatically, sometimes people make mistakes, like editing a document, then fat-fingering a mass deletion in the document during the save process and not noticing it until a month later. Again, RAID doesn't protect you from that, but a proper backup system does. And finally, it's not unheard-of for RAID systems of any kind to just fall over. Even a RAID 10 is a whole lot of redundancy all running on a single drive controller; if that drive controller does something very silly -- and I've seen it happen, we've personally shipped systems to data recovery outfits for this -- then you can end up with garbage on all of your drives. There are two different concepts at work here. "Redundancy" and "High Availability". RAID uses redundancy, but it belongs under "High Availability" -- it's a system that's designed to still be mostly available even if it suffers a hardware failure. But strictly speaking it does not provide for redundancy of your data.