I keep forgetting the diskutil commands so I've dumped them in the this post.
To use dd in mac you have to remember to umount the volume first (as everything gets auto mounted)
mount - find the path for the mounted volume i.e /dev/disk3s1 (diskutil list) is also useful
sudo diskutil unmount /dev/disk3s1 - unmount it
sudo dd bs=512k if=imagefile.img of=/dev/disk3
