Rsync Backup between two ReadyNAS - is overwrite required on first backup?

With StephenB 's help, I've properly set up 2 NAS boxes as follows:

- RN528X:  Primary NAS

- RN424:    Backup NAS   (Rsync Backup programmed on this NAS, i.e., this is the "pull" NAS)

Works perfectly.

However...  now I've added a 2nd backup NAS, an RN212. This NAS already has a full copy of the Primary NAS on it. But of course, I have scheduled the backups for each share to capture future changes on the Primary NAS.

I began a Pictures backup (6+ TB) and observed that the RN212 Backup NAS is (apparently) re-copying all of the data from the Main NAS, even though all the Pictures data is already on this RN212 Backup NAS. That is, appears to be overwriting the existing RN212 Backup NAS data, instead of performing a DIFF to look for differences. I'm judging this based on hovering the mouse over the RN212 Backup NAS's "Pictures Backup" Status ("In Progress" w/ green circle-icon). See picture below. The data on this RN212 Backup NAS was copied over using Windows Explorer over a long period - it was not done using Rsync. 

Is there no way around this on first-time Rsync backup overwrite? Seems like such a huge waste of time and needless disk wear, if the RN212 Backup NAS is indeed re-writing all this data. I can hear the disks spinning furiously, so it's definitely doing something. I'm looking at 2 1/2 days to complete first backups, so if there's a way to short-circuit this I'm all ears. For example, in the Backup routine, is there a way to tell it not to overwrite exact duplicate data?