George's Weblog

I had four WD RED 4TB drives and wanted to get better performance out of them with a mirror but also wanted the ability to replace a drive if (when) one failed.

Indications from online searching said I could create either a RAID 10 or 0-1. I'm not sure if one is better than the other. I went with the 10 configuration since I'm already familiar with that on my TrueNAS.

Trying to do this from the Disk Utility always failed with an error message saying the RAID couldn't be created. The only type of array I was able to create out of two mirrored pairs of drives was a concatenated array. That still gave errors but appeared to have worked. But that's not going to give me any performance benefit.

Instead, the solution is to do everything from the command line. So here we go....

By the way, my disks are in an external cabinet attached via Thunderbolt 3.

The four disks are disk2, disk5, disk6, and disk7 (use 'diskutil list' to find yours). Make sure you get the correct disks as they will be repartitioned and reformatted.

Let's create the first set of mirrors from disk2 and disk5. I named the set vdev0.

# diskutil createRAID mirror vdev0 JHFS+ disk2 disk5

Now we'll create the second set of mirrors from disk6 and disk7.

# diskutil createRAID mirror vdev1 JHFS+ disk6 disk7

Now do a 'diskutil list' and find the disk numbers of the newly created mirrors. Mine were disk8 and disk10.

Finally, create the striped set of mirrors. I named mine WDRED.

# diskutil createRAID stripe WDRED JHFS+ disk8 disk10

That's it. if you do a diskutil AppleRAID list you'll see the two mirrors and also the striped set which should now be mounted.