I back-up my photos every couple of weeks and I found out that if the laptop and the first HDD have the same directory structure, the rsync can figure out the diff and only copy over photos that are not yet backed up. This also let's me archive photos from my laptop to my first HDD at any time and rsync still picks it up correctly.
It works like this
rsync "$source_dir_laptop" "$source_dir_hdd" "$backup_destination" -ah --info=progress2 --delete-before --backup --backup-dir="$delete_backup_dir"