Have you tried removing the partition on the external USB drive, repartitioning then reformatting? It appears in certain cases the My Cloud doesn’t properly mount an external USB drive. In my case I had one USB drive the My Cloud OS3 firmware simply wouldn’t mount. Removing the partition, creating a new one, then reformatting solved the issue.
Another way to test is to use another USB hard drive or flash drive formatted for a Linux file system to see if that device is mounted.
One can see my experiences with one particular drive not being mounted including some log data at the following thread:
https://community.wd.com/t/this-is-absolutely-unacceptable/149432/8
