3 of 4 ST31000528AS Drives doing bad in NV+ ?

First, never use rsync to do an initial backup, it is way too slow. It's strength is that it synchronizes the changed portion of a file between two devices. The process of checking each and every file is exceedingly slow when there are no files on the other end. When using rsync between two NAS units, it is recommended that the first backup be done using NFS not rsync and then change the type to rsync and run it again to verify the backup. It typically only takes a few minutes for rsync to run in this mode.

I would recommend in your situation that you use copy/paste to do the backup in smaller batches so as to not stress the drives and the NV+. I would start with the folders containing the most critical information first. Typically this is financial information and family photos/videos which cannot be replaced, and then work to the files that are less critical and/or may exist in other locations.