As a guy who started out on a CDN-88, then dabbled in the Technics movement, and finally settled on Traktor/VCI-100, I can say this problem has a simple solution: Take two laptops!
All joking aside, I've had almost every piece of kit in my rig fail on me at one point or another (Microphone, mixer, CD player, hell I even had to take a Numark CDmix1 apart at a gig once) i've learned that the only viable solution is to bring a spare of anything you didn't already bring two of (speakers). I bring two laptops, one iPod, two SM-58s, two amps, and two hard drives to every gig I dj, unless its a 10 minute or less drive to my house. Period. This ensures that should one part of the system decide "tonight's the night I don't want to work anymore", you are not SOL.
At the end of the night, I like my clients to be happy and I like to be paid; by bringing backups of everything, I ensure a successful gig every single time. It's worth it. Trust me.