When creating a RAID 1 mirror set, do you need to have both disks available or can one be added later? I'm planning my transition to Mac OS and I have a pair of mirrored drives I would like to move to my future computer. Since they are currently formatted as NTFS they will need to be reformatted during the transition. What I would like to do is create a RAID 1 mirror set with only one of the drives, copy the data from one drive to the other, and then add the remaining drive to the RAID set. Does anyone know if this is possible to do, or will Disk Utility require me to have two drives to create the RAID 1 mirror set?
This can be done from the Disk Utility app or from the diskutil command line.  Be warned that you should definitely make a backup before doing anything.  I had an issue where the diskutil appleraid enable mirror disk1s2  command I ran wrapped the volume in a RAID set but the partition is now called Apple_RAID_Offline and no longer accessible.  Ironically after restoring from backup to a new volume (I'm still trying to figure out if I can recover that original), I was able to create the one-sided mirror just using the Disk Utility.app.  I simply dragged the volume into the RAID window and it gave me a warning that it would be changing the volume to a RAID set but that the data would not be lost.  Not sure what happened with the command line attempt but it's slightly disturbing.  In either case once the one-sided mirror is created you should be able to drag another partition into the Mirrored RAID set to complete the mirror.  or use the diskutil ar add member &lt;disk&gt; &lt;RAIDset&gt; command.  

In your case it should be easy to create the one sided mirror first and then transfer your data to the new volume and then format the ntfs disk and add it to the mirror set.