To make one from a CD:
Insert the CD and use the “mount” command to figure out its device name. We want to unmount the CD without actually ejecting it.
sudo umount -f /dev/disk1s1s2
dd if=/dev/desk1s1s2 of=mynewimage.iso
I tried using `hdiutil eject /dev/disk1s1s2` to then eject the CD but that failed me. So, I rebooted and ejected the traditional way (drag the mounted CD icon to the trash in the finder).