Easy encryption with Disk Utility | Macworld: Launch Disk Utility and choose File -> New -> Blank Disk Image. In the New Blank Image window that appears name your image, choose a size for it (something roomy enough to accommodate all the files you’re likely to move to it), select 128- or 256-bit encryption from the Encryption pop-up menu, and select Sparse Disk Image from the Image Format pop-up menu. I suggest choosing sparse disk image because your disk image will take up only as much storage as the sum of the files within it. So, even though you’ve created a 2.5GB image, if you’ve only flung a single 300MB file into it, that’s the size it will be. (It can hold up to 2.5GB but won't stretch beyond that boundary.)