Proably cause something is running on the background.
See if these help… But it will depend on your needs.
http://community.wd.com/t5/My-Book-Live/Runaway-mediacrawler-The-Ultimate-Solution/m-p/578720/highlight/true#M23756
http://community.wd.com/t5/My-Book-Live/MBL-Sleep-Mode-and-firmware-02-32-05-046/m-p/523307/highlight/true#M17112