Did you try re-partitioning and then reformatting the drives that were not recognized by the My Cloud? I found that one drive, a Toshiba 1T external USB 3.0 drive wouldn’t be mounted by the OS 3 firmware. I re-partitioned and reformatted the drive in a Windows 7 PC and it was suddenly recognized and properly mounted by teh My Cloud OS 3 firmware.
There is a lot more on my experiences, including lots of logging information, in another thread starting with this post:
https://community.wd.com/t/this-is-absolutely-unacceptable/149432/8
Bottom line in my case was the default out of the box partition and formatting the drive came with, while it was mountable under the v3.x My Cloud firmware, and worked in all other computers (Windows Vista, 7, 10 and Linux), simply wouldn’t’ mount properly under the OS 3 v4.x version My Cloud firmware. Re-partitioning and reformatting of the external drive fixed the issue.
