IIRC the extra pin on a 4 pin motherboard header may report if the fan has totally stopped, like a failure line, to tell the mobo to shut the CPU off. I've got an old 4 pin fan here from a '94 vintage ...

Building a PC isn’t easy for everyone, but if you’re someone who’s planning on doing it soon or just a PC enthusiast who’s passionate about PCMR, then this is the perfect blog for you. In this column, ...

On page 4, there is an "IDE Power" connector. It looks a lot like a 4 pin CPU fan connector, but it (obviously) provides different voltages that are appropriate for 3.5" IDE drives (5V, 12V) and the ...