Hey guys this may sound like a stupid quesiton, but I really don't know the answer to it.
Should I power off the computer when plugging in a case fan into the power supply? The reason I would prefer to keep the computer on is because it takes a few minutes to boot up again. I would rather just plug it in and watch the temperature monitoring software to see if the fan is improving the temperature.
Yes, shut down the computer and pull the electrical plug from the wall outlet (mains). No point in risking the consequences of having a bad fan or a bad connector. Also no point in risking fingers accidently touching something.
When we have computers on the bench we run them all the time with open cases, BUT anytime you are actually adding something or touching the insides for any reason, you should have it off and unplugged. Too many things can go wrong!
Personally, however, I wouldn't worry about plugging a fan into a Molex connector provided there is easy unfettered access and when you move the wire, it's not pulling anything else which might short out to a piece of metal on the case. I would NOT however plug the fan into a MoBo fan header while it's on under any circumstances.