macOS Sierra : dd to USB is very slow and can't seem to use /dev/rdisk
I installed the same image last night on a RPi3.

You need to unmount the disk first. Then you can write to the disk using sudo. 

diskutil unmountDisk /dev/rdisk2
sudo dd bs=1m if=~/Downloads/your_image.img of=/dev/rdisk2
diskutil unmountDisk /dev/disk2


I'm on macOS Sierra, too.