More good info thanks Tacit.
As Artie ask as well:
What is the controller fails, then you may not even have one good drive? Keeping things separate usually only one drive dies and you quickly replace it.
That controller on the Raid, sounds kind of scary in that it's decoding and then coding back garbly gook, and it that goes screwy the whole house of cards could fall down?
The purpose of a RAID array is to be resilient in the face of failure. If any part of a hard drive fails--heads, platters, controller, anything--the information on it is gone. A RAID array can tolerate a failure and keep going.
Most RAID arrays these days use pretty standardized parts. They're not really a house of cards; they're quite robust. Years ago, I installed an array for a client of mine, an advertising agency that was at any given time working with many gigabytes of files for a catalog they produced. The array had nine drive slots, two controllers, and two power supplies. It was designed so that if a controller, a power supply, or any two hard drives failed, you would still have all your data. (If one hard drive failed, it would just keep on going; if two hard drives failed, it would go into an emergency mode where you could still read your data but you could not write new data until you replaced the failed drives.)
As Virtual1 says, they're not backups; they're designed to make your data accessible even if something fails. You still aren't protected from doing things like accidentally deleting a file. Even with an array, you still need backups. Think of an array as a hard drive that is very, very difficult to kill.
