Time machine cannot make a copy

My Imac with Maverick is connetcted to the WD My Book for Mac 3 tb disk. All the time i get the message that Time Machine was unable to make a back up on WD Backup. From time to time though, it makes back ups every hour, for a couple of days and then suddenly it gives the same message, TM cannot etc. I removed password security and tried some other options, but it does not work properly. Any suggestions as to how to resolve the problem?


Do you encounter the same behavior if you disable the built-in sleep timer features of your WD My Book?