I have my time machine partitioned and backed up with Drobo.( I have a Mac Pro mid 2010 running 10.8.4) I recently had one of the four hard drives in the Drobo fail. I replaced it and now the Drobo says that everything is healthy, but Time Machine says I need to reformat the disc. I've tried to repair it with disc utility, but it always fails, saying the disc is apparently "read only." How do i reformat it?