Set Up Server Backup Forces Full Disk Backup of Non-System Disk

I have a Windows Server 2012 Essentials setup with 4 x 1TB disks. I needed to reconfigure my backup settings because one of the disks had grown and so did the backup disk up to maximum capacity (In other words, it was full). I decided that I wanted to leave out the whole disk all together but Set Up Server Backup forced the backup of the disk with the following message: Server Backup requires that you back up this item to ensure that you can restore the operating system in the event of a catastrophic failure. I didn’t understand the reason till I remembered I installed MDT (Microsoft Deployment Toolkit) ...