Can I lock an external drive?

A friend has just had a break-in, and - not content with stealing the things he'd worked hard to get - the robbers put the entire contents of his hard drive online.

On hearing this I changed my settings so that my MacBook would lock a few minutes after sleep.

But what about my backup drive, a FireWire external hard drive, which contains all the same information. Can I set this as locked, and to open only with my admin password?

I'm using an Intel-based MacBook running Leopard, I think it is - the latest OS, anyway.

 

How do I do that visual thingy, please?

I looked at the TrueCrypt page, but it seemed rather nerdy. It tells you what it does, but not what steps you take to use it. I'm a simple soul, and need this type of information.