I use rsync for something similar to this; it might help you too. I use ‘-P’ which makes the destination be like the source (so you never accidentally delete files in the source) and --dry-run will list what changes would be made without actually changing anything, effectively givingg you a directory diff. I’m sure there’s a flag you could use to say “I only want to create new files in dest if they’re not in source, but not delete or change anything” but I don’t know what it is off the top of my head.