That all depends on the brand of laptop. Some Gateways replaced the LS-120/floppy module with a 2nd HDD. IBM, I believe, has a 2nd HDD for the CD drive module. There is no standard for this - you need to explore each brand and model.
The best generic way of getting a 2nd HDD is an external Firewire or USB 2 case for 3.5" drives. That's what I use on my new HP dv1000.