Recently one of the drives on my My Book Pro failed and so I bought another WD Black drive of the same 4GB capacity. The rebuilding started as it should but after about 15 or 20 minutes, the process stops and the light on the existing drives goes out but the light on the new drive turns red. However the Mac continues to “see” the existing drive and can be read. In fact if I click and open a file or folder the Rebuild continues and goes for another short time. After doing this many times I am now up to 61%.