his example had two 2tb drives in raid 1 that would be 2tb real space wich btw is plenty for most people if you need more i suggest a promise or g-tech 4 bay raid5 all 2tb drives meaning 6tb space
But, again, with a g-tech 4 bay Raid 5, you lose the ability to mix and match drives and upgrading the array in the future will not be nearly as easy as with the Drobo.
As for 2TB being "plenty for most people", I think it was Bill Gates who once said, "No one will ever need more than 637 kb of memory for a personal computer."