I have had many issues using the MyBook drives as boot drives for the Pi. The only way I have been able to get it to boot was:
Start from a Power Off state for all devices.
Plug the MyBook into power.
Plug the MyBook into the Pi's USB port.
Plug the Pi in, and boot.
If the system goes down or needs to reboot for any reason, the only way I was able to get a successful boot was to repeat the above steps. In my personal experience, the MyBook line is unsuitable to use as a boot drive with the Pi. I have found, however, that the EasyStore line of drives does work well if you want to stay with WD.
A new release is out and the links to the RPI image have been updated on our website. This build should fix the boot loop issue, and supports more models like the Pi 400.