"rsync" is free and specifically designed for this purpose. It's usually installed on Linux distributions out of the box.
https://linux.die.net/man/1/rsync
Given 2 servers:
rsync -avP
[email protected]:/path/to/source/folder /path/to/destination/folder/
You can also do it within a computer as well if you have multiple drives.
rsync -avP /drive1/source/folder /drive2/destination/folder
Subsequent runs of the command will check modified dates and file checksums. From there it will only copy the changes. You can run it once a day or however often you want.
It's installed in OS X as well and can be used in the Terminal.