Prep SD card with image

format the card

http://www.makeuseof.com/tag/write-format-manage-sd-card-linux/

dd if=/dev/zero of=/dev/sdc

WARNING: this will wipe the card, make sure the target is correct. of is target and /dev/sdc is a disk in this case

Download the image to copy

Find a copy of the image you want online and start download. In this example Kali:

https://www.offensive-security.com/kali-linux-arm-images/

prep the image

1. unzip the image

   xz -d kali-2017.01-rpi2.img.xz

2. unmount sd card
   linux

   `lsblk # verify disk

   sudo umount /dev/sdc1`

   mac diskutil list diskutil unmountDisk /dev/sdc1

3. copy image to sd card
   sudo dd if=kali-2017.01-rpi2.img of=/dev/sdc1 bs=1m

4. cleanly unmount sd card
   diskutil eject /dev/sdc1