Which firmware version My Cloud do you have? The newer v2.x or the older v4.x?
On the older v4.x when I had problems with one particular USB drive failing to mount properly on the My Cloud I found I had to look at two log files; kern.log and wdnas.log. See my posts with log info in the following thread where I discussed my issues as we tried to track down why my USB drive wouldn’t get properly mounted on the My Cloud. There are several other commands one can issue to try and track down exactly how the My Cloud see’s the USB drive and how it mounts.
https://community.wd.com/t/this-is-absolutely-unacceptable/149432
