If you click on the alert or alarm icon next to the USB icon on the Dashboard, does it indicate that the My Cloud couldn’t mount the external USB drive?
Some of us are having trouble with certain external drives being mounted by the My Cloud post OS 3 firmware updates. There are one or two other threads discussing this issue with no firm solution as to how to fix it other than try a different USB drive. Here is one such thread discussing USB mounting failures.
https://community.wd.com/t/usb-my-book-not-recognized-reporting-0k-available/96850
