I think you should split your 5Tb LUN into, say, 5 1Tb ones and then create a separate DSSU against each LUN. Next, as you got your brand-new DSSUs, you should create a storage unit group and add these DSSUs into that storage unit group. Symantec recommends "round-robbin" priority for that sort of storage unit groups. Finally, you configure backup policies with storage unit group as a backup destination and voila you have fault-tolerant D2D2T solution. It is should be mentioned that the best practice is to spread multiple DSSUs across a numberof media servers to prevent media server failure.