Magic Mirror

MagicMirror allows you to easily create a mirror copy of any volume (or part of) mounted on your Mac towards any other mounted volume or available in the network.

Only the modified files are effectively copied. This results in a moderate hard disk access and the process lets you continue working with your Mac.
Important Note : MagicMirror is not Magic Mirror Backup (tm) by Pensamos Digital, Inc. which is a great product too.