Foldersync is a free console utility for Mac OS X, Windows and Linux (both 32 and 64 bit). Foldersync was built to synchronize and back up files.
Incremental backup. Only modified or new files copied from a source folder to a destination folder (for example external USB drive). Software detects files changed by the user or other software in source folder and copies them to the destination folder (backup). Incremental backup is much more efficient than backing up the entire source folder every time.