Is there a way to automatically update a folder in external hard drive without deleting missing files? I have a 250GB macbook air. I do a lot of photography, so my SSD gets filled up with photos really quickly. I've been using a 2TB external hard drive to archive the older photos that I no longer need to store in my SSD, moving the folders with the photos manually to the external hard drive, and then deleting them in my SSD. I was wondering if there is a way to do this automatically? The process should backup all the sub-folders in my Pictures folder to the hard drive, but without deleting the sub-folders that I have already deleted in my SSD.
If you are not afraid of using Terminal, you can run

rsync -a --progress /PATH/TO/Pictures /Volumes/EXTERNAL/Pictures/