I have an external hard drive (A) that I write files to all the time. Every so often I copy it to a second external hard drive (B) so I have two copies in case anything goes wrong. How do I only copy the changes Ive made to (A) usually being additional files I've created without having to pick them out before copying to (B). The only option I seem to get is "Replace" or "stop" on my Mac. If I replace, it takes ages as it completely re-writes (A) on to (B) and its over a 1gb. Im using OS X El Capitan and the Time Machine facility is no good as my Macbook Air's storage capacity is not large enough for this amount of data unless I'm missing a trick!!! Can anyone help please?