Create a RAID 1 with only one disk? I have a MacBook with an SSD, so space is limited. 

I've been using a single external drive to store my overflow, but would feel a lot more comfortable if it was on a RAID 1. So, I've bought an 2 bay USB 3 drive case, and an additional hard disk (same exact disk). What I want to do is create a RAID without having to the move the data on the external disk to a new temporary home, if at all possible.

I have:

Disk 1 - contains files.

Disk 2 - empty.

What I would like to be able to do, is create a new RAID 1, with Disk 2 as the only member. It will therefore show that its in a degraded state.

Copy all my data from Disk 1 onto Disk 2 (which is RAIDed). Reformat Disk 1, add it to the Enclosure, and make it a member of the RAID, and have the OS silently replicate the data from Disk 2 to Disk 1, so that it becomes a mirrored set.

Is this possible?

Or do I need to find a 3rd disk to use as a holding area for all of this?
The short answer is no. In any case you should place your data on a third drive, make your raid on drives 1 &amp; 2, then copy data from drive 3 back to the new raid (drives 1&amp;2).

Please note that even raids can (and will) fail too and you should keep a backup of the raid.