Find your USB drive:
diskutil listThen unmount it
# Replace <disk2> with your disk name, rdisk is faster?
diskutil unmountDisk /dev/r<disk2>Copy the ISO to the drive (pv shows progress bars, 1MB at a time):
pv FreeNAS-9.3-STABLE-201602031011.iso | sudo dd of=/dev/rdisk2 bs=1048576killall Dock
Ctrl + Cmd⌘ + Space (ref)