I have a 3 pin 12 V Fan, rated at 2000 RPM max. Its nominal voltage is 12 V which will make it spin at full throttle. That's what is happening when I plug it in the motherboard. However I don't like that, because the fan is making a lot of noise. I would like to lower its speed to about half. I have figured out I should supply it with 7 V instead. There are methods online here, here and even a famous video on YouTube here demonstrating the procedure.
I've read a lot about this and I'm unsure about whether this is safe. I use a modern and pretty good and reliable 80 Plus PSU. So I'm skeptical now and I want to ask here to understand better. What we have to do is pretty much shown on the picture below.
Instead of supplying the fan with 12 V, using the yellow 12 V pin and the black ground pin from the Molex cable, we instead supply it with 12 V for VCC and 5 V for "ground". So the fan sees voltage difference of 12-5=7 V, but shouldn't there be a path to 0 V ground? This is the part where I'm confused.
Is this secure, is this possible?