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Using OS X, I have a 1 TB hd. I have an empty 1 TB that I want to use to create a mirrored array. All the instructions I can find about creating the mirrored array require copying the existing hard drive (which is my boot drive) to a third location, creating the RAID then copying the data back.

Why?

Is there not way to me to just tell OS X that my existing boot drive was already part of a RAID and that 1 drive failed and I wnat to re-create the RAID with the new drive?

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hardware RAID controllers operate on drives, and assume their RAID sets are setup before data is written to the drives. some software RAID implementations (eg Linux) can operate on partitions, so they may be able to do the kind of thing you're asking about. – quack quixote Feb 23 '10 at 3:44
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Not exactly. There might be some software out there that can do this...but think about it this way -- you will have a copy of your harddrive if you do the third drive thing. The ultimate backup.

RAID, even simple mirrored, actually changes the format of the data on the drive. I'm not exactly sure of the details, but it does end up wiping your partitions and such when you add a RAID volume to a non-raided drive.

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Ok, thanks. So both drives actually are changed to support the raid. If after creating the RAID 1 drive fails, I will be able to replicate that to another drive without wiping out the original drive again, correct? Thanks. – Michael Shnitzer Feb 23 '10 at 3:37
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afaik, in hardware RAID1 it's just adding the RAID controller's metadata to the drive (tho i'm not sure of where, and whether this would force repartitioning). in RAID0 or RAID5 the actual data is written to the drives differently (you're striping across multiple drives). not sure if software RAID1 requires writing similar metadata. – quack quixote Feb 23 '10 at 3:41
@quack yea, you're probably right in that RAID 1 does't change much on the drive, but it changes enough to wipe out partitions (oops back in the day that taught me the hardway). @Michael -- yes, as long as you don't fiddle with your controller's knowledge of the volumes, you can just put in the other disk and it will rebuild the array. Don't shuffle them around though or change their jumpers. – Nextraztus Feb 23 '10 at 3:46
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Yes, you can. It's just not Apple supported. http://www.macosxhints.com/article.php?story=20041013121106176

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looks viable. i'll give you +1 if you edit your answer to include the steps. (link-only answers may suffer from broken links in the future.) – quack quixote Feb 23 '10 at 5:09
I posted the link because the comments there are helpful too. MacOSXHints isn't going anywhere. :) – churnd Feb 23 '10 at 11:46
true, but that doesn't guarantee they'll maintain that link. – quack quixote Feb 24 '10 at 12:00
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