use rsync to backup content and ensure that files deleted on source is not deleted on destination but copied to backup directory
example
rsync --verbose --progress --stats --compress --recursive --times --perms --links --delete --backup --backup-dir=../backup\ of\ deleted\ content/ --exclude=backup-dir/ rsync\ source/ rsync\ dest/current\ nas\ content/
The –exclude=backup-dir/ is properly not needed
If you are not interested in moving deleted files from the source to a backup directory but just want to ensure that files that have been transferred one from the source to destination are not deleted on a subsequent execution of the rsync script then remove all with backup and the flag –delete
e.g.
rsync --verbose --progress --stats --compress --recursive --times --perms --links rsync\ source/ rsync\ dest/