So is this correct?
When you add an external drive onto an E* receiver, you're adding to the total drive space (i.e. if your internal drive is 300GB and you add a 500GB external, you now have 800GB storage).
When you add an external drive onto a D* receiver, you're "replacing" the internal drive (i.e. in the example above, you would have 500GB total storage).
When you add an external drive onto an E* receiver, you're adding to the total drive space (i.e. if your internal drive is 300GB and you add a 500GB external, you now have 800GB storage).
When you add an external drive onto a D* receiver, you're "replacing" the internal drive (i.e. in the example above, you would have 500GB total storage).