When I say computer "back-up" I'm not talking about an external HDD or any number of software support and online services available to keep your data safe (although this is certainly important to think). I mean how you, as a teleworker / or an independent contractor, to prepare for the inevitable breakdowns in the computer hardware that is going to happen. This is particularly important for independent contractors, that often double as the problem solver of its own technicians.
I keep a cemetery of computer classes in the basement with sufficient spare parts to improvise something in an emergency (most I also can hijack computer of children in a hurry). I keep all my important files backed up online and in an external hard drive, so easily I can access them from a different computer.
This is useful in emergency situations not too. Recently, when a new customer had only training software for Windows XP (children team has no XP!), I had to dig through my treasure of technology. The old portable powder with a broken monitor is connected to an external monitor - and taking up precious space in my dining room. But it is better to lower or delay because we couldn't get done training.
Relying on old junk, is an easy solution for someone like me that computer needs rarely go beyond text and navigation processing for the web. But those with greater computing needs and/or employees, what are you doing?