Red light on WD My Book Live Duo

I have exactly the same problem. Message was “failure to mount volume”. I too am using it as one 6tb drive. The diags state that the B drive is bad.  In my case, I had less than 3tb of data,  I would like very much to recover this if I can. Since it was configured as a single drive can I buy an exact replacement drive. What steps would need to be taken to install/configure?