When you shoot multiple cards, usually the workflow is to unload them once you get back to home base. This is usually enough, but if you want to be absolutely on the safe side (or just want to empty your cards) you have two common solutions: use a laptop to transfer the files or use something like the $219 WD My Passport Wireless hard drive that comes with an SD slot.
Reader Sasha Stojkovich just sent in this clever tip that enables a backup from practically any card to practically any portable hard-drive. The secret sauce? An OTG card reader with a USB hub built in.