What software can I use to keep my backed-up photos in sync?

For many years now, I've been using an external hard drive as a backup for my photos and videos. I also back files up on DVD-R. In both cases, I'm finding it difficult to keep track of what's been backed up. Is there software that can scan my external drive, plus my main computer's Pictures folder, and show me where there are differences? This would help me see where photos are missing from the backup. Because the backup process is manual (I just drag folders and files from one drive to the other) there's a lot of room for human error. Plus it's time-consuming. I'm sure there are some good tools out there. I'm currently running Windows 10. Thanks!

--Submitted by Mike M. from Seattle