For that you'll want to look into encryption utilities. TrueCrypt is one option which runs on multiple platforms, so you're less likely to find yourself in a situation where you can't access the data. There are other options, but without knowing more about your setup, including your operating system, it's hard to say. However, have you ever considered placing it under lock and key?
is there some program out there that would let me password protect my external so that regardless of wat computer i hook it up to, it still prompts a password to be entered?