Backing up a NAS to multiple volumes (Ready NAS 314, Firmware 16.10.8)

I have a hard disk housing attached to a rear USB on my NAS.  I have 4 back-up jobs that backup different shares and the destination is that housing and what ever drive I have inserted in it.  I us a bare drive (NTFS), run the jobs, and then store the drive offsite.  I was considering adding a second drive and wanted to check here if my concept is correct.

My proposed process is to alternate the destination drive each time.

1. I ran the last back up a week ago to the original destination drive.
2. If I run the backup jobs today to the new drive, it will back up ALL files to the new drive.
3. When I run it a third time (to the original drive), It will compare and only backup those files that have changed since the last time the original drive was used.
4. When I run it a fourth time (to the new drive), I assume that it will compare only back up those files that had changed since the last time the new drive was used.

Or is my recollection incorrect and the back up does not check the destination and only that a file has changed since the last back up job was run regardless of which destination drive was used?

Thanks in advanced.