External hard drive encryption

I've got a complete backup of my main PC on 1.5 TB external hard drive that I carry around with my laptop so I can have access to all of my files while I'm on the move, however it has just dawned on me that if someone nicks my external hard drive they now have access to everything!

Hence I'm looking for a way to encrypt my external hard drive. I'm after something that is:

• Secure (if I need to carry around a USB dongle to keep the key on so be it)


• Fast (the performance of the drive should still be reasonable)


• Cross-platform (I regularly use other peoples computers - Sometimes they are not windows based and might not even have internet access, however I still want to be able to access my files)


• Cheap (preferably free / open source!)

Answer

What about Truecrypt? (http://www.truecrypt.org/)
Secure: Yes.
Speed: Depends on how you encrypt your drive.
Cross platform: Yes.
Free: Yes, and open-source.