Setup mirror volume

First, let me correct a misconception you have. A Mirror is not a duplicate of one volume to another, a Mirror means a single volume, with two or more disks.

You need a third boot disk to make the changes you desire, but yes you can do it.

copy all data off the external. All data needs to be on the internal drive
Startup from a third disk or thumbdrive
Initialize the external disk, which will wipe all data from it.
Convert the internal to SoftRAID format.
Convert the boot volume to Mirror and select the external disk
Convert the second volume to Mirror, and select the external disk.

Now you can restart to the internal and let the rebuilding happen in the background.

OK understood re-how a mirror works. Can I convert JUST the Photos partition on the internal HD to SoftRaid format and will it keep the data currently there?

It is not possible for two disk drivers to talk to the same disk. So no, all volumes on a disk with a SoftRAID volume will be controlled by the SoftRAID driver.

Thanks for the correction. I thought I could use an external HD to mirror the internal HD as a mirror pair.

If I convert the internal HD to SoftRaid will that erase the system files and data on that drive? Can I just convert the Photos volume?

Note: I only need to mirror the Photos partion, the System partion is cloned.

You do not need to mirror the system. But SoftRAID's driver will control the entire disk.

Thanks for all the help. Using your instructions I did manage to get it to work within a test environment. Unfortunately in the live system the internal drive is a fusion drive and so I could not convert it to SoftRaid. I don't want to "defuse" the drive, so will now have to look at alternative solutions.
Thanks once again for the assistance.