I use my iPod as my external HD - 20GB, Firewire and it plays MP3s! 🙂
But it's not 300GB...I would recommend Seagate, there are also some drive enclosures that you can buy and then swap out the hard drive by simply opening it up, and placing a new hard drive in it - standard hard drive works in it. My buddy did this, saved him a couple bills on total hardware costs and is able to easily upgrade the hard drive when he needs to.