Try performing a 40 second reset on the 3TB My Cloud. Such a reset shouldn’t remove any user data. Plug the USB drive into a Windows PC and run a error check on it just to be sure there are no errors with it if you haven’t done so already. Try using a powered USB hub attached to the My Cloud with the USB hard drive attached to the USB hub.
Officially there is no way to roll back the firmware. Unofficially there may be one or two ways to do so. See the following thread for more information on how to roll back certain My Cloud units.
https://community.wd.com/t/wd-my-cloud-v3-x-v4-x-and-v2-x-firmware-versions-download-links/148533
