Yes you can boot from a portable hard drive depending on what the connection type is. I know that you can boot to USB, and I think with OSX you can also boot to a firewire 400 or 800 drive. However, unless your connection is Firewire 800, your speed difference between the external, and running it as an internal drive is going to be much slower. 800 will be slower as well, but it won't be as obviously noticeable. As far as I know, it isn't possible to boot of an esata connection.