FreeFileSync is a free and open-source folder comparison and synchronization program available for Linux, Windows, and Mac. I swear by it for all my disk sync operations. It is advert-free, powerful, fast, light on system resources, and provides a side-by-side visual comparison of the source and destination folders. This way the user can review the changes and uncheck or define filters for files and subfolders you don't wish to sync. On my Linux system it routinely scans well over a half million files in under 5 minutes. I'm always happy to endorse quality FOSS applications.