How do I attach My Book to My Could Mirror

I’ve just brought the 8TB My Book Hard Drive and attached it to the back of the My Cloud Mirror, like I have a 3rd party (Touro) hard drive. The Touro is showing as attached but My Book is not. When I plug the My Book directly into the PC I can access it. What am I doing wrong or have I missed something here?

The firmware is 2.12.127 (last update: 9 June 2020)

Hi @78378,

Please contact the WD Technical Support team for best assistance and troubleshooting: