Two My Book Essentials wit WD smartware

I have two 2 TB WD Essential drives, one the USB 2.0 kind and the newer one the USB 3.0 kind. Both have WD smartware and I use the the older USB 2.0 as data backup only. The newer USB 3.0 drive is replacement for a failing  drive used for file storage (music, pictures, video). I want to continue to use this drive as storage only with thedata to be backed up to the USB 2.0 drive as before. I can’t seem to figure out how to accomplish this with WD smartware. The newer drive does not appear in the pull down list of drives available to be backed up, rather it appears as a drive that can be backed up to. Any ideas, or should I return the drive?