Some of us have experienced similar issues that have been discussed in a couple of threads (see below). In my case I had two external USB hard drives (one from Toshiba and one generic docking station) that wouldn’t be properly mounted by the My Cloud v4.x firmware. The solution for the Thoshiba drive was to delete the partition(s), then reparation the drive then reformat for NTFS. I used a Windows 7 PC to do the reformatting/repartitioning. I talk about it in the following post:
https://community.wd.com/t/this-is-absolutely-unacceptable/149432/23
Other threads where this issue is discussed.
https://community.wd.com/t/my-mycloud-doesnt-recognize-some-usb-hard-drives/152488
