Time machine backup has stopped working

I have been using the time machine back fine for a few weeks.

Now I added my wifes macbook air to do a time machine backup.

This worked fine.

However mine now will NOT backup to the WD mirror. I get this message.

What the heck is this ?

Screen Shot 2015-01-09 at 12.04.52.png

I can see the time machine backups in the WD mirror backup folder if I browse network.

I have tried deleteing the drive and adding it back but then I get this message.

Screen Shot 2015-01-09 at 12.15.59.png

SO what is going on, there is no disk to eject.

Hello there,

Have you tried resetting the drive? If stills the same after a reset, please contact WD Support directly for assistance.

WD Contact info: