Windows – How to set up two existing disks with identical contents as a single mirrored volume in Windows 7 without losing data

I have two data disks that were, heretofore, in a mobo RAID configuration in Windows 7 (Ultimate, 64 bit). They are now separate AHCI disks, visible in Computer Management. How to I go about making them a single mirrored volume in Windows?

Note: The data is backed up up on two other separate disks, but it's a fair amount of work to do a restore (over 120'000 files, and I have to reset permissions).

Note2: Currently the two disks are identical, and I can use the content of either one for this.

Edit 1

I've converted one volume to dynamic and deleted the other, and rebooted just in case, but "Add Mirror" is still disabled. Also, trying to create a "New Mirrored Volume" only offers the unallocated disk to include in the mirror.

Edit 2

The problem seems to be that, although the two disks are identical, the disk to be mirrored needs to be smaller than the disk to which you want to mirror. So I just decreased the partition size by 50 MB and then Add Mirror was available and I was able to mirror (in my screen shot) Disk 3 to Disk 0. I suspect that I needed only a much smaller difference, perhaps 1-10 MB, but it's not worth my time to experiment to find the smallest difference that will mirror.

Edit 3 (2014-03-31)

Although making the existing disk smaller fixed the problem originally, having subsequently needed to re-mirror the disk at a later date, I then resized the disk back to it's full size and was able to successfully add the mirror – with identical hardware as when this question was posed. So it seems like a bug or quirk in Windows which may have been resolved.