I didn't need the extra hard drive on the last trip, but with 9 days of shooting ,with the 5Ds R for five days and the 1D IV for 4, of 512gb I had 50gb of space left when I got home. For the last few years I have been traveling with the external, but it is nice to have all the pictures on one machine so you can easily edit while sitting at the airport, on the plane or wherever. You can do it with the external to, but IMO it makes it just a bit more cumbersome.