I travelled on motorbike for several months on each my last photo-trips, so I was looking for compact back up solution just as you are.

I was toying with the idea of tablet or some smart external drives, but fortunately, I stepped away from it and decided for an ordinary 13' Macbook Pro and USB powered external drive. All my photos are on the Macbook and duplicated on the external drive. When I visit friends on my journey, I always ask them if I can temporarily copy photos from USB drive to their computer, which acts as an emergency off-site back up until I get home.

Advantage is that laptop can do much more than just hold copies of your photos. You can do some work, keywording and pre-editing as well. Todays laptops are small and light, if you can get hold of a good laptop, there's no reason to pay for some other single-function device.

Obviously, ideal solution is taking more SD cards. They are cheap and tiny. I use CompactFlash, which costs more and need space for thousands of RAWs, so I carry a laptop.