Downloading to a USB drive requires you to have a device into which to plug the said USB drive, but you are apparently not taking such a device with you. If you are going on vacation to a place that that has the facility (i.e. not in the wild) you could go to an internet café and plug a USB drive into their computer to download your images from your cards. It might be a good idea to take a USB card reader with you in that case.
To me, the simplest and cheapest way is to get a few spare cards for your cameras, (obviously you need both CF and SD cards, based on your cameras) and cycle them through. You should be taking spare ones with you in any case in the event of a card failure at a critical moment. Then download them to your computer when you get home. You can get a compact, but robust card holder for storing and organizing them. That way you have spare cards for future occasions, so it's an investment in the future!
cheers, TREVOR
The mark of good photographer is less what they hold in their hand, it's more what they hold in their head;
"All the variety, all the charm, all the beauty of life is made up of light and shadow", Leo Tolstoy;
"Skill in photography is acquired by practice and not by purchase" Percy W. Harris
