There's plenty of software out there that'll do the trick... both commercial and freeware... and most likely something could be done in Terminal also... but a solution for someone who doesn't want to buy any additional software or get into Terminal is to use disk images. You can launch Disk Utility and click on "New Image." It'll ask where you want to save this new image, and you can the external hard drive. Give it a name, assign it a size (you could go with one or two large disk images, or several smaller ones depending on need), and then under "Size" areyour encryption options "none" or "AES-128". Choose "AES-128" and click "Create." It'll ask what you want your password to be, and then the disk image will mount... files and folders can then be added to that mounted disk image, and then the mounted disk image can be "ejected" and you' be left with an encrypted .dmg file... To retrieve data from it you'd just double click the .dmg file, give it the password (you can save the password in your keychain on your own computer if the main concern is the drive being stolen or "borrowed" by the wrong people, so you don't always have to tell it the password, but it's more secure to not save it in your keychain), and the disk image mounts... I'm sure this isn't the slickest way to go about it, and may not even be the best, but it can give quick and fairly easy protection, and can be used either for short term while shareware or commercial software solutions are considered, or long term for as long as you (or your friend) are happy with it...
