Mac: Write .img file to a SD card

1. Use diskutil list to determine the driver identifier of the SD card

2. Unmount the SD card.e.g. diskutil unmount /dev/disk3s1

3. Write the .img file to the SD card, e.g.
sudo dd if=filename.img of=/dev/rdisk3 bs=1m


Ref:
osx - Writing .img file to SD Card from a Mac - Raspberry Pi Stack Exchange