Hey everyone, Back in October/November 2014, my beautiful late 2013 (16GB RAM, Intel i7, everything maxed out) iMac's fans started behaving very strangely. The fans would spin up to full speed when the machine was doing nothing, Activity Monitor reported 2% CPU load, and iStat reported that temperature readings were fine. More strangely, early in the morning, after the machine was sleeping all night, the fans would spin up all by themselves, spin up, spin down, spin up, and continue until I shut down the machine (this isn't pleasant at 5AM when you're trying to sleep). Naturally, I tried a PRAM reset, SMC reset, nothing. I started noting that the problem happened in Windows (gaming partition) as well, which ruled out driver/software issues. I've taken the computer to the Apple Store, and they replaced the logic board. When I got it back it turned out that the bluetooth was now broken, so they replaced that. Now that I have it back, the fan is starting to act up again, behaving like before. Anyways, it seems like the computer is malfunctioning completely and it's a real drag. An Apple employee once told me that it could potentially be a USB device which is causing the fans to act like this: but my iMac's fans act up when any USB external HD is plugged in: is it true that a USB device could be at fault for my Mac's erratic behaviour? I already opened another topic regarding my experience with this iMac in the Apple Store and I was looking for some technical advice on the matter before I take it back, yet again. Is there any way I could personally fix/troubleshoot the issue? I find it suspicious that they've replaced the logic board yet the problem is persisting. The problem is particularly acute since my warranty expires in roughly two weeks and I don't want to be stuck with a malfunctioning iMac. Thanks in advance.