Chances are it is indexing your media.
Do some searches on this forum as this topic has been covered quite a bit. There are also different solutions. It will all depend on your needs.
Here are some:
http://community.wd.com/t5/My-Book-Live/MBL-Sleep-Mode-and-firmware-02-32-05-046/td-p/523307
http://community.wd.com/t5/My-Book-Live/Blinking-Green-Light/m-p/577282/highlight/true#M23578