There have been a number of discussions on the My Book having trouble with the My Cloud that can be found using the forum search feature, magnifying glass icon upper right. For example the following recent discussion:
http://community.wd.com/t/connecting-a-my-book-to-the-usb-3-port/213944
You will need to ensure the My Book is formatted in a format (NTFS for example) the My Cloud supports.
https://support.wdc.com/knowledgebase/answer.aspx?ID=15124
Also see the following WD Support Knowledgebase article:
https://support.wdc.com/knowledgebase/answer.aspx?ID=17340
And this Knowledgebase article:
