Time Machine v western digital my book live fail second backup

HI I am using iMac 27 inch, running Lion.

I used Western Digital Mybook live as my time machine backup.

I follow the instrustion step by step, its work fine.

However, when I restarted my mac. Then its start fail to do continues backup.

I can't do any backup anymore. Its always show fail.

Anyone have idea how to deal with this?

Thanks.