FreeFileSync

FreeFileSync is a folder comparison and synchronization software that creates and manages backup copies of all your important files. Instead of copying every file every time, FreeFileSync determines the differences between a source and a target folder and transfers only the minimum amount of data needed. FreeFileSync is Open Source software, available for Windows, Linux and macOS.

This is the "GPL release" which uses the sources as published by the author, as opposed to the "FreeFileSync Donation Edition".