macos - How to encrypt all—or part—of a USB hard drive that can be accessed by both Linux and Mac OS X?

I bought a new USB Hard Drive that I want to use as a back up for my Linux (Ubuntu 14.04 LTS at the moment) and my Mac Mini (running Mac OS X 10.9). I can do this perfectly at the moment, but I want to be able to encrypt the files that are on the hard drive. I know I can do this if the hard drive was to be used by either machine exclusively, but unfortunately it’s used by more than one machine running two different operating systems.

Is it possible to do this? I’d prefer a whole drive encryption, but if an “encrypted folder” solution is the only way—or is the better way—then I’d like to know how to do it.