If you really want to be paranoid when you're out shooting, then you need two independent systems for backup. The problem with only a laptop and 2nd hard drive is that if the laptop fails, then you're unable to backup anymore files to the hard drive. In essence, you're now dead in the water until you get home.
So along with a laptop, it's recommended to additionally use an Epson P-2000 or P-4000, a Jobo Giga-Vu Pro, or some other sort of indepentdent device to transfer your files to. That way, if wither the laptop or the other device fails, then you still have one available to use.
Of course this means bringing more gear, but if you're on an extended trip to some remote location, it's better to be safe than sorry.