I have an NZXT Source 210 Elite and the back fan only has a 3 pin connector. The wire is already routed under the GPU so I don't want to unplug it from the motherboard. Since it doesn't spin very fast I decided to plug the fan into the molex too. Is it alright to leave it plugged in to the motherboard and molex at the same time?
The three-pin fan header on the motherboard may be spinning the fan slowly because of a BIOS or software setting, which in turn could be basing the RPM of the fan on a temperature sensor somewhere. If you're going to plug the fan into a four-pin Molex anyway, do not also plug the fan into a three-pin motherboard fan header.