In the past this has been an issue, a USB drive name being appended with a number and not being accessible or not being properly mounted. The workaround is to remove the attached USB drive then access the My Cloud Dashboard > Shares page and delete the USB share if it is still listed. Then reconnect the USB drive and access the Dashboard > Shares page and rename the USB Share back to the original name.
There are other discussions that suggest going into the USB database file (autoMount.db) on the My Cloud and removing the entries for the USB drive and then reattaching the USB drive.
https://community.wd.com/t/my-cloud-cannot-not-mount-share-on-4tb-my-book/168409
