To answer my own question, which may help anyone else: I bought a Vosonic VP2160. It’s a very simple tool. Only two buttons are present: power on/off and copy. IF you insert a card (16-in-1 card reader) and press copy, it makes a new directory and duplicates the card to that directory. So any files (not only pictures) will be copied. And with a weight of 230 grams its very light. I bought it for a mere 144 euros.

Got a similar one, albeit the VP-3310. It includes a MP3 player but using it is a sure way of draining batteries. It has no display to show the photos, but considering that a display drains batterries even faster then MP3 that is a good thing!

Typically the battery will last for 2 hours or so. Tranferring a GB will take 20 or so minutes. So typically your battery should last 5 or 6 GBs. The batteries are rechargable so if you bring you charger with you there should be no problem. The size of the hard drive is your own choice. The hard drive is just a standard drive (not sure if it is desktop or laptop standard, my guess is laptop) so can be had in varying sizes and writing speeds.