You mirrored a 2TB disk onto a 4TB disk. That means that only 2TB of hte 4TB disk are availble. The rest is unallocated.
You need to increase the partition size on the DataVolume partition on your 4TB disk so that there is no unallocated space on teh 4TB disk. It’s a bit Linux intensive, so research it properly before doing it.
easier is to just follow the instructions you’ve been given already and use the debrick guide. You’re not the first person to do this, so read the guides of people who have been so kind to give you step by step instructions. use Guide 2 http://community.wd.com/t5/My-Book-Live-Duo/GUIDE-Debrick-a-MyBookLive-DUO/td-p/544860
