The recent issue of MagPi (Issue 50 October 2016) includes an article about Etcher. Etcher is a free app for burning an image to a SD Card. Downloads are available for Mac, Linux and Windows. I don’t know much about the company but the Etcher app seems to work very well (and fast) on a Mac.
If you wish to look thru the source code before running Etcher, the code is available at: