The 651BD, 751BD and 752BD can only recognise the MBR disc partition system. This is limited to a maximum disc size of 2 terabytes. Larger drives generally use the GPT partition system which cannot be read by the 651BD, 751BD or 752BD. Some drives have both systems which is why the players will see 2TB of the drive.
Both players will read larger drives over network so if you need larger capacity, this might be the best course of action.