How to encrypt a single folder on my external hard drive? [duplicate]

I have used encfs for such a task.

It allows you to mount the folder to encrypt onto an empty folder. After that, everything you put under the mount point will be encrypted. When you unmount the folder, your crypted data is no longer visible, until you mount it again.

There is also a GUI, in the package cryptkeeper.

You might consider TrueCrypt. There is a nice GUI, I believe.

In the meantime since the original writing in 2011, a few things have changed. TrueCrypt is no longer available as the project maintainers have suddenly declared the software as insecure. A security audit was performed and a fork of the software, now called Veracrypt, has continued the development.