Folder Lock for external Hard Disk?

The only way to really "lock" anything in a computer is to encrypt it. You're thinking of it like a folder that you can hold closed, but the proper term for it is actually a "directory"- it's a list of where on the hard drive (physically) you can find those files. There really isn't a good, portable way to encrypt a directory because that has to be built-in to the operating system. The best you can do is use a program like 7-zip to create an encrypted archive (zip folder or something similar) and then carry that around.