[Premo]

Pro Tools software doesn't 'require' you to have an external drive per se, but it's highly recommended and needed to have a 'supported' system in Digi eyes.

Many of Digi's suggestions are common among most DAWs and pro audio manufactures. Such as Oxford chipsets and such. I recently saw the same suggestions on RME website. And they try to safeguard against all situations.

I've seen many people record with USB drives just fine, I've used a LaCie d2 Triple interface many times successfully over USB. But it might not work for you, and thats why Firewire is recommend because of it's better bandwidth.

Esp with laptop drives it's going to be difficult for the internal drive to keep up with the amount of audio processing going on. You can try it, but esp with 4200 adn 5400 drives you'll most likely hit a road block.