During a rainstorm, so much water falls on your roof that you could fill several rain barrels from a half-inch of rain. So you won't need to block off the existing downspout.

You only have one downspout for the whole house? I'm not a roofer, and I can't see the layout of your gutters, but it seems like unless you have a very tiny roof you might be doing yourself a favor by adding a downspout. Obviously it needs to be downhill from the highest point of a gutter, but it should still work--ask a roofer!

Thanks for the follow up.. I think I have it figured out....Picture a large rectange for my house, with gutters on the long sides of the house, the peak in the middle going longways as well...... Since the gutters actually travel quite a distance for the home, I am going to drop the pitch of the gutter along one side of the house for about 15 feet and put in a new downspout there so I can collect the water at the desired side of the house and just leave the others alone. I think that will be plenty for me...