For some reason the My Cloud OS has trouble with the way certain USB hard drives are partitioned. While you may have reformatted (or formatted) the USB drive, did you try deleting the partitions and repartioning the entire USB drive?
In my case with a Toshiba USB hard drive the My Cloud OS3 firmware refused to properly mount the drive and it would show 0K. There are several other threads that go into this particular My Cloud USB hard drive 0K issue. In my case repartitioning then reformatting solved the problem and the drive is now recognized by the My Cloud. I detail my experiences and eventual solution in the following thread:
https://community.wd.com/t/this-is-absolutely-unacceptable/149432/8
