I have a 2TB internal HDD, and 2x 2TB external HDD in USB caddies. So 6TB would be the console's physical maximum.
But, if you have ConnectX enabled and connected to a PC with plenty of space, you can go way over 6TB. If you decicate a 8TB drive solely for ConnectX, you've got 14TB space for games. I've come to the conclusion while testing most games work fine through ConnectX, it's only a bit slow with loading. Mind you some games require local storage and crash on ConnectX.