Since we are working with 9 Gig drives, lets stay with that and see what happens with RAID 1. The R letter kicks in with this standard and Real protection from drive failure happens. In RAID 1 you get a byte for byte copy of the other drive. One drive goes poof, and is D.O.A. the other drive is still singing.
The bad news is the cost in both money and using up the inside of the computer. Two 9 Gig drives give you exactly 9 Gigs of storage.
This is because each drive is a mirror image of the other. In fact this is popularly known as mirroring.
Ok. So you think life is good because you have a full time backup happening with a second hard drive.
What happens if the controller working the two drives gets served a digital death certificate? Well, the controller might just slip quietly into no-more-work land.
And it might get real annoyed at the fact it is being killed. When a controller does give a last act of defiance, it could scramble the data, perfectly mirrored of course, on both drives. Now you have a redundant pile of garbage for data.
The solution is known as disk duplexing. By having a separate controller for each drive, if one gets taken out, it cant force the other controller to misbehave, leaving one drive with the data.
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