Instructions were taken from here: http://www.macosxhints.com/article.php?story=20041013121106176
The “enableRAID” command to change an existing active volume into a RAID slice doesn’t always work (diskutil sometimes refuses to do this). Here’s a safer and surer way to turn your boot disk into a RAID group with minimal down time and without installing OS X on the new disk.
The basic idea is to turn the new disk into an empty RAID slice (this seems to always work) and then copy the boot disk onto it. Boot up into this one-disk RAID group, and then turn the original boot disk into the second RAID slice. This is safe because you’ve already successfully booted into the copy before you overwrite the original. In more detail:
1) I started by using Disk Utility to erase the new drive I was adding. (Or OS X may offer to do this for you when you first plug the drive in). Make sure the format of the drive is OS Extended (Journaled). I named the new drive “BootRAID”. I then quit Disk Utility and from a terminal window ran “diskutil enableRAID mirror /Volumes/BootRAID” NOTE: The name of the drive is NOT relevant, as it can be changed when the process is compete.
2) I then restarted Disk Utility, clicked on the new drive and used the Restore option to copy the boot disk onto “BootRAID”. Note that the new drive has two entries in the list of volumes (slice name and RAID group name) and only one of these has the Restore option.
3) When the copy was complete I selected “BootRAID” as my boot disk in System Preferences and rebooted. This reboot was the only downtime.
4) Now I was booted into the one-disk (degraded) RAID group and I again used the graphical Disk Utility and dragged the old boot disk into the RAID
group. This erased the old boot disk and started the mirror “repair”.
NOTE: If you have issues just dragging and dropping the disk into the RAID group, you may need to erase the drive first!!
The repair copied the mirror disk onto the original disk, and a few hours later I had an intact mirrored RAID group as my boot disk.