And running Linux or formatting your hard disk won’t help.
A hacker can build a backdoor on your hard disk by targeting and reprogramming the controller, a tiny computer of its own that makes sure the hard disk works.
First, the hacker needs to gain (remote) access to your computer, and he or she has to be pretty good. That means that you don’t need to lose much sleep over it yet, but when it happens, you’re toast.
Unless, for example, you keep your computer offline afterward and make sure it can’t be accessed via powerline networking either. Would you be able to tell that there is a backdoor on your hard disk?
Read more here.