The issue of certain USB hard drives not properly mounting is well discussed here (see threads below). For some the solution is to delete the partitions on the USB drive, create one single partition and format in a My Cloud supported format (NTFS, FAT32, etc.).
https://community.wd.com/t/this-is-absolutely-unacceptable/149432/23
https://community.wd.com/t/usb-my-book-not-recognized-reporting-0k-available/96850
https://community.wd.com/t/my-cloud-cannot-not-mount-share-on-4tb-my-book/168409
Additional discussion on this issue can be found using the forum search feature, magnifying glass icon upper right.
