There's something I'm confused about: Why can a bad device connected to a motherboard (such as a wifi card, optical drive, hard drive, etc) crash a mac during OS X runtime and not during startup? According to apple troubleshooters in Apple-GSX, a Kernel Panic or System crash may be produced by a bad device, so that to diagnose it Apple advises you to: 1: start off a known good OS X volume such as an external hard drive (this would mostly rule out software) and if the problem continues 2: disconnect as many cables as possible from the motherboard/logic board: WIFI card, hard drives, optical drive, Card reader etc. and then test it like that (using an external monitor and external hard drive). My doubt is: What does apple exactly mean by a system crash? I understand it's a freeze or kernel panic during OS X execution and not an spontaneous restart during start up (before logging in), in fact, apple calls these crashes during startup "intermittent shutdown". In fact, if the unhealthy restart occurs during startup, the apple troubleshooter doesn't advise you to remove all those device cables and test. Instead it simply asks you the typical pram and smc resets then try with an external drive. Why can a bad device connected to a motherboard crash a mac after log-on and not during startup? In what way can a defective device produce a crash?