My Book Essential cuts in and out

Be sure to always safely eject the My Book before disconnecting the USB cable, since this may cause data corruption and unstable performance. You can try running a test using DLG to verify if the hard drive has any hardware problem. Check the link below for the steps. 

http://wdc.custhelp.com/app/answers/detail/a_id/940

http://wdc.custhelp.com/app/answers/detail/a_id/5316