Copy photos from iphone to sd card?

I'm looking for a way to offload photos and video that I take from my iPhone 4S onto an SD card.

I'll be traveling overseas without a computer for a few weeks, with an iPhone 4S. I probably won't have wifi very often and when I do I expect connection speeds will be very slow, so it's really not practical at all to upload lots of large photos and video to any internet-based storage. What I need is a way to transfer all the photos & videos I've shot during each day to an SD card. I have a 500 GB "photo storage device" that accepts SD cards and will be bringing this along. This has proved a great system for offloading lots of photos and large videos on previous trips - at the end of every day, before turning in, I remove the SD cards from still cameras and video cameras, copy the cards' contents to that 1/2 TB storage device, reformat the cards, and pop them back in the camera fresh and ready for the next day's shooting.

Now that I'll be taking along my iPhone, I want to be able to offload photos/videos from that, too, to the same storage device.

I have the "iPad camera connection kit" but IIRC, that's a one-way trip, the other way (copy stuff off a card and onto the iPad - don't know if it even does that with an iPhone, and in any case I need to move files in the opposite direction).

How can this be done?

Thanks.