We usually store our photos, documents, and more on a single hard disk--or, increasingly these days, a solid state drive (SSD)--but there's always the nagging worry that the disk will fail, taking all your work and memories with it. Backing up using Time Machine, Super Duper!, or CrashPlan, say, is a good way of reducing this risk, but there is another: RAID.
RAID can be incredibly complicated, but it's extremely worthwhile--one of the things it can do is to mirror the contents of one disk completely to another, all the time. While cloning your hard disk using Super Duper!, for example, is something that might happen once a day, with a RAID system, every bit of data that's written to one disk is simultaneously written to the second, so that if one drive fails, you have a perfect copy of everything it contained on the second. (And optionally, if you replace the failed drive, everything will be mirrored back across to it automatically.)
The especially good news is that a mirrored RAID setup like this, once it's configured, appears to you and to your computer as a single disk, so it's as easy to use as a single disk--defining it as a backup target, say, or simply dropping files onto it like with any other disk--but just safer.
You could use a mirrored RAID system like this to store important archives or current work projects, or you could give yourself extra protection against lost data by using a mirrored RAID drive as the one you back up to using, for example, Time Machine. That way, you're backing up, but your backup itself is doubly protected against failure. This is what we'll be doing here, but you can use a mirrored RAID disk for anything you like that requires a bit more data security.
You can buy external RAID hard disks that have two or more hard disks inside a single box, but you can also easily make your own using the built-in tools in OS X. Standalone RAID systems tend to perform slightly better, but for home or small office use the difference is negligible. Here, we're going to show you how you can repurpose a couple of old hard disks you might have lying about (or picked up cheap on eBay) to create a mirrored RAID system. Even if you're not sure about how reliable your old hard drives are, with this easy, free method, you can use them with a bit more confidence.
Gather your drives
For this technique, you'll need two spare drives--usually external, but you can also use internal drives, like if you have an old Mac Pro or G5. It doesn't matter for the process of creating a mirrored RAID how the drives connect, what size or capacity they are, or what kind of drives they are. It doesn't even matter what format or partition map they are, as they'll be wiped anyway--just ensure there's no data on them you want to keep. Note that the size of your mirrored RAID will only be as big as your smallest drive. In this example we have a 160GB drive and a 500GB drive, meaning the finished RAID will be 160GB--the rest of the 500GB drive's capacity will be wasted.
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