8 channels of audio is really not that much to be recording, even at 96/24. The problem with just using one hard drive is that if you are writing those 8 files all at once, while reading the programs that you are running off the same disk, then maybe you could run into problems.
There are plenty of bus powered hard drives out there, if you want to go that way, just pick one. Here's one from LaCie (
http://www.lacie.com/products/product.htm?pid=10833) that would do the trick, there are some firewire ones out there if you must have firewire, but USB 2.0 is *technically* faster than firewire 400. Not trying to start a debate over the two.
Either way, 8 channels is not a lot, i would calculate that you would need a sustained 18.5 (ish) Mbps write to your disk (24 bits/sample * 96000 samples/sec * 8 channels). Most 5400 rpm drives (assuming your laptop is one of those or higher) shouldn't have trouble doing that for extended periods of time.