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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.


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

2 Answers 2

up vote 2 down vote accepted

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
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

Yes, you can. It's just not Apple supported.

Article pasted below for posterity

Read the rest of the article for the how-to -- and please read the dislcaimer before you start doing something like this to your system! These instructions also assume some Terminal knowledge...]

DISCLAIMER: I take no responsibility if you choose to follow the steps below. I make no garauntees that this will work for you, however it did work beautifully for me. You may potentially lose all of your data, so I strongly suggest making a backup copy before trying this!

  1. Install a new hard drive
  2. Boot off Mac OS X Install CD #1
  3. Install a minimal OS on the new hard disk:
    • Click 'Customize' when it asks what type of installation
    • Uncheck everything but the BSD subsystem box
    • This install takes maybe 10 minutes
  4. Reboot and hold the Option key to get the boot loader menu
  5. Boot off the new hard disk
  6. Open a terminal and su to root
  7. Run the following command: diskutil list:
    • Look for the old disk in the output, will probably be disk1
  8. Next run diskutil enableRAID mirror disk1:
    • The disk will disappear from the desktop for about 30 seconds, then re-appear
  9. At this point, I reboot the system and hold down Option while it reboots
  10. Now select the old disk to boot from
  11. Log in and open the Disk Utility app in Applications -> Utilities
  12. Select the RAID volume and the RAID tab
  13. Drag the new disk to the RAID
  14. Click Rebuild:
    • Rebuilding usually take 1-2 hours depending on the size of your disk
  15. Drink a beer as you now have a mirrored boot drive

There should be an easier way to do this. You could theoretically boot into single user mode off of the Install CD, and bypass installing the OS on the new drive. However, diskutil seems to be broken in single user mode; I believe this is a known bug.

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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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