For all those saying RAID isnt a backup solution...
Well no it isnt, but it does lessen the need for a backup solution due to hardware failure which is the most likely way you will lose your data. Quite honestly if someone breaks into your home/work and steals your computer, well they have it dont they? Not a lot of times a backup would be much comfort now that your computer is gone...
As for me, I dont backup much because backing up 12TB isnt exactly convenient, cost effective, time sensitive, or did I mention cost effective and time sensitive... I mean it would take me over a week just to perform the backup to a locally stored RAID and any means of the backup for 12TB wouldnt be practical at all... What 3000 Single layer DVD's? 1500 Dual layers? 6x2TB drives? 12TB over USB2.0 hard drives? Forget it...
At some point RAID5 and so on is a perfect means of backup because the process of actually backing up isnt practical, the larger your data source becomes... You think major manufacturers/banks/corporations backup to USB hard drive? Please... They multi site disaster recovery / business continuance to other RAID storage... And maybe just maybe, they write to tape backup if they get the time...
So if you are suggesting that the average user buys an offsite alternative home/office to store their precious data on a server that replicates/synchronizes to the original source data, well that just isnt practical at all...
In short, RAID is actually a form of backup when other means prove less effective due to cost, source data size, or just plain time to perform...
Not to mention in cases like my storage array, I have exhausted all of my hard drive connections internal and cant connect any more drives... Backing up to an external source would be next to impossible...
BUT, I do backup offsite to webserver storage, the most important data for an extra layer of protection... That only pertains to about 20GB of data or so for things like family pictures, emails, accounting data, and taxes.