I use a lot of great apps as I code my own apps and take care of my content. The SyncBack app has been around for quite a few years now. It’s all about synchronising files between different folders and media. It offers advanced strategies for syncing, can run in the background and can even handle sync with FTP sites.
I use it for backing up data to my encrypted USB drive and a lot of other things.
It comes in a freeware version and a pro version, and as with all great apps, I always encourage users to go for the pro version. It will support the vendor to continue supplying great apps.