I used to do this on a pre-unibody Macbook Pro. The main drive still had OS X, I used that only when I was in coffee shops or airport lounges. But I also had a

I used to do this on a pre-unibody Macbook Pro. The main drive still had OS X, I used that only when I was in coffee shops or airport lounges. But I also had a 40GB SSD expresscard that was bootable that was fully encrypted that I used for my personal use. It worked really good, you just slipped in the expresscard and held down the option key when you started up, and chose the other drive. I carried the card on my person when I traveled, if someone stole my machine I didn't lose anything, and I suppose if it was searched at a border, nobody would know it wasn't my main OS.