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Creating Mirrored Raid on Startup Drive

by markw10 / December 23, 2010 4:47 PM PST

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:

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.

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by mrmacfixit Forum moderator / December 23, 2010 9:50 PM PST

Everything that is in that document is true, not sure why you would think otherwise.

Yes, the drives in the Raid will be erased as part of the RAID building process.

Yes, you cannot use the Start up volume as part of the RAID as this would remove the OS and the RAID building Software.

No, you cannot use the Start Up drive to create a RAID that you want that disk to be part of. (See above)

The way it is done in OS X Server, and on the Win platform, is to use a special startup disk that will boot the computer and use whatever drives you require to be part of a RAID. Once that is completed, you use a regular Installation disk to install the Server OS onto the RAID so that the RAID disks both contain a working copy of the System and will boot the computer.


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Time Machine
by markw10 / December 24, 2010 4:31 PM PST
In reply to: RAID

I'm using OS X Snow Leopard non-server edition so I'm not sure if that makes a difference. 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.

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Had you mentioned backup
by mrmacfixit Forum moderator / December 24, 2010 10:05 PM PST
In reply to: Time Machine

in the first place then Time Machine would have been the obvious choice.

A RAID system does not take the place of a Backup routine.

We have a dozen servers with RAID arrays and all of them are backed up to tape every night.

There is very little performance hit with Time Machine, after the initial Full Backup.


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