MyBook saying it's "read only"

I have been using my WD MyBook for TimeMachine back-ups for my MacBook Pro for a few months now. Suddenly last week, I am getting an error message on TimeMachine saying that the drive is “read only” and that it cannot do the TimeMachine back-up. I’m not sure how to change this. Please advise.

I don’t know about Mac but in Windows the first thing to try is to delete the driver for the external drive and reboot.