It has been working fine - I can here the disk spinning but no lights on the front. I have switched off the Mac and restarted. I know that the cable is fine as I daisy-chained another firewire drive from the My Book enclosure and that mounts fine.
Welcome to the WD Community.
Depending on the My Book you might have a USB connection on it. If that’s correct, please try using that connection to test the unit.