I've always started by wiping the card so it's fresh. The job completes, but I can never get the card to mount afterwards. I've been using dd on the command line with various buffer sizes. So the day has come when I want to come back to working on the first project and I've been unable to write the saved DMG image to an SD card. I wiped the card to use it for another project with plans to eventually come back to the project saved on the image. This saved it as a DMG file that mounts nicely on the Mac when I double-click it.
In my cheap attempt to not use up too many SD cards while working on multiple RPi projects, I created an image of a configured RPi's SD using MacOS X's Disk Utility (GUI).
