I have a WD Easystore 4 tb drive that I use to backup my WD My Cloud drive. When I plug the Easystore into the back of the My Cloud it will mount and I can run the USB Backup jobs I have created. Also, while connected to the My Cloud I can access the drive on my MacBook.
When I ejected the Easystore and tried to then connect via my MacBook USB the drive shows up in Disk Utility but will not mount. I ran First Aid and it came back ok but still it will not mount.