I use PGP (Pretty good privacy) to do this...........
You encrypt any file, whether it's a whole CD, a hard drive or just any individual file, then assign a PW that then has to be entered before it will open.
They had some major trouble , getting banned in America and sold out...the software used to be free but they updated it, gave the NSA the backdoor to the encryption (so the security people know how to get into anything created in the later versions) but you can still get the older, free version...and it's very simple to use
Have a google for PGP free version, it should be around.....or just google 'encryption software +free' and you'll find quite a few progs that will do what you want