It looks like your one disk is got some problems. There is a very good chance it will need to be replaced. But, you are correct to want to try to format it and see if that repairs it. Since we don’t know where the problems are in the disk - which partitions have the problems - and since you have a functioning disk B with the OS on it, we can strip the disk A down to a blank disk without partitions.
The best way to do this is to attach the disk to your computer running linux and use some linux commands to first repartition it into one big partition.
Alternatively, you could connect the failed disk via USB to the USB port on the back of the MBLD. Connect as external storage. Then you could use this guide http://community.wd.com/t5/My-Book-Live-Duo/GUIDE-Attaching-external-storage-to-MBLD/td-p/865319
Once you have the disk into a blank condition, then you can put into to the MBLD and let the MBLD partition it into 4 partitions and then format it. Basically you will be following the instruction in teh MBLD users manual on how to replace a failed disk.
