Hehe, no USB, you've got an oldie ;P
Well, in this case, I'm unsure of what you could actually do. I think in the end, you'd do more damage than anything. I mean I bet you could make a converter if you get the connections(from the looks of it, it's just a standard 40pin connector with 2 of them for power... But you'd have to have this sitting outside the machine, and all...Just not the best enviroment for a harddrive or laptop ;P Oh, and I don't think you could have more than one harddrive either. Many laptops I don't beleive are made to contain more than one.
Also, some other problem that might arise...I'm not sure the exact pinouts for 2.5 drives, but it looks like they only use 12volts, and I'm pretty sure that 3.5" drives use both 12 and 5 volts... I might be wrong here...