Depends on what features are important to you, but FreeFileSync (available on Windows Mac and Linux) meets my needs, i.e. synchonosing data between macOS and Windows via an external SSD drive.. I suspect it will come up as a popular alternative to Syncback on the AlternativeTo site suggested above.
Download FreeFileSync 14.11. FreeFileSync is a free open source data backup software that helps you synchronize files and folders on Windows, Linux and macOS.