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.
Note too that this is a handy way of recycling old internal drives--maybe you recently swapped one out for an SSD and it's sitting in a drawer?--either by slotting them into a dock or mounting them in a drive enclosure. (Mine is from Macally, and Other World Computing sells them too.)
Start building your RAID
Connect your drives then launch Disk Utility (/Applications/Utilities) and click on either of the two disks you want to build into a RAID. Click the RAID tab at the top of the right-hand pane, and name the single drive you'll create in the RAID Set Name field. Ensure the RAID Type dropdown is set to Mirrored RAID Set.
Add drives and set options
Drag the drives you want to use for your RAID setup from the list on the left into the space on the right. Click Options below this space and check the option to have the RAID set automatically rebuilt.
Create the RAID
Click the Create button at the bottom right, double check you have the correct drives specified, then hit the Create button in the confirmation sheet.
Use your new RAID drive for backup
Depending on your system, as soon as the RAID set has finished being built, you might get a prompt to use it as a Time Machine backup; if this is what you want to do, click Use as Backup Disk, but otherwise click one of the other options. For example, you might wish to use a different backup system, such as Super Duper! or Carbon Copy Cloner.
Though there are some optional steps below, you're essentially done now. By backing up to this mirrored RAID drive, you're giving yourself some extra protection; now your main drive can fail and even one of your external backup drives can fail at the same time, and you'll still be able to recover your data. No backup system is guaranteed--all you're doing is mitigating risk with each additional layer of protection you add--but this is a good step in making catastrophic data loss less likely.
Inspect your setup
Before you started this process, you would have had two external drives mounted on your Mac, and showing up on your Desktop and in the Finder Sidebar. Now, it looks like you only have one, but that's because the system is treating those two drives as a single disk. The two drives are still there and connected, but you don't address them independently; you chuck everything at the new RAID set you just created, and the system takes care of mirroring your data to both drives. Have a look in Disk Utility, and you'll see how things are configured.
Break up the RAID set
If you want to redeploy your RAID drives elsewhere in the future, you can easily break apart the RAID system. Select the set in Disk Utility then click Delete.
As the confirmation dialog says, all that will happen is that you'll get two drives mounting on your system once you do this, each of which contain an exact copy of the other. New data added to one will no longer be mirrored to the other, but you can now eject one and use it for something else.