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markw10 · Dec 24, 2010, 04:49 AM

I have a Mac Pro that I want to create a mirrored RAID set in.
I have a 1TB Hard Drive that is the main system drive.
I just added a 2nd 1TB Hard Drive that I wanted to use along with this first hard drive to create a mirrored RAID set.
I used the following document to help with the process:
Server Admin 10.6 Help: Creating a RAID Set Using Disk Utility

I have opened up Disk Utility and think I may know how to do this but a few things concern me.
First, it states that it will erase the contents of the drives it's using to mirror. I had the idea it would simply duplicate the data onto the other drive. Is it true that it will delete the data on the current drive?

Also, it states that you can't create a RAID with the startup disk. Is that true? I currently have a RAID in this system using the disks in bays 3 and 4. I purchased two 1TB disks and created a SERVER DRIVE raid and this drive is shared with all computers on the network as a data drive. I had no issues setting it up but of course both drives were empty when I created this RAID and also these are not the system/startup drives.

Is there a way I can create a RAID with the main Macintosh HD system/startup disk and also do it without erasing the data from the main drive?

Thank you for your help.

SSharon · Dec 24, 2010, 11:36 AM

I have a mirror RAID setup as boot drives in my desktop MDD so I know that is possible. Unfortunately, I can't recall whether I had to erase one or both of the drives when I first set it up over 5 years ago. If you go to disk utility and walk through the motions it should give you a clear warning that it will erase the drives. The more I think about it the more I think I had to have fresh drives.

besson3c · Dec 24, 2010, 11:46 AM

There are indeed ways to add a RAID mirror to an existing drive, I've done this in FreeBSD, but I'm not surprised that this is not made possible in Disk Utility since it is generally encouraged to do this with new drives.

If Disk Utility won't enable this you could try the diskutil command line counterpart.

besson3c · Dec 24, 2010, 11:49 AM

From the diskutil manpage:

enable mirror|concat device
Convert a non-RAID disk partition containing a resizable filesystem (like JHFS+)
into an unpaired mirror or single disk concatenated RAID set. Disks that were
originally partitioned on Mac OS X 10.2 Jaguar or earlier or were partitioned to
be Mac OS 9 compatible may not be resizable. Ownership of the affected disk is
required. diskutil enableRAID is a deprecated synonym for diskutil appleRAID
enable.

It looks like you'll need to run diskutil appleRAID enable on your drive(s) first before creating the mirror set in diskutil.

markw10 · Dec 25, 2010, 04:29 AM

Thank you for the information you provided. The more I'm looking at it instead of going the RAID route I'm considering Time Machine. I have used that successfully on other systems and it would back up hourly and if I delete a file it won't replicate onto another hard drive. I am sure Time Machine slows down the system some but I think it may be a better option for me to use than RAID. We also have an offsite backup which is done weekly.

besson3c · Dec 25, 2010, 11:17 AM

Yeah, TM is indeed a better backup solution all-round. RAID is for increasing the availability of your system - i.e. it needs to remain online at all times.

Moderator · Jan 21, 2011, 03:32 PM

I agree Time Machine over a software raid 1 like you are describing. the main prob with the raid is that if something is deleted, or corrupted..boom so is the mirror copy. With Time Machine at least you have versioning.

I too wish Time machine could be configured to run at certain times, so it isnt constantly eating cpu cycles. I have used chronosync successfully as an alternative. It can be scheduled and can keep deleted files. But Time Machine is just so easy thats its difficult to recommend against it.