WD my book 3TB USB 3

Hi, as a troubleshooting step, try connecting the power cable directly to a wall socket. Connect the USB cable directly to the Mac and avoid using USB extension cables or USB hubs. To test the My Book use the Disk Utility app. Check the link below for the steps. 

How to test a drive for defects or problems on a Mac 

http://wdc.custhelp.com/app/answers/detail/a_id/866