I have a late 2009 21" iMac. It gets so hot in the summertime that I had to buy a 6" clip-on fan to cool down the area of iMac. It is a cheap product that I bought at a bargain store. I positioned and clipped the fan in the back area of the stand where the iMac sits on, it titled at an angle where the fan can blow at the top part of the iMac. Turned it on at low power. So, for a while, I was typing away on a project on the iMac. Then the keyboard was not responding to my keystroking. Puzzled, I used the tab key to switch to a different app, not responding. BUT... the mouse is responding and I was able to move the cursor around on the screen, open or close the app. I checked to make sure the keyboard USB is firmly plugged in. Still no response from the keyboard with typing. I rebooted. iMac was working normally. I resumed back to my project typing. Clip-on fan is still blowing at the back of the iMac. Few minutes after reboot, the keyboard is not responding again. I thought this is odd. To be on the safe side, I turned off the clip-on fan. Rebooted the iMac and it's working normally (although I noticed screen flickering/dimming every once in a while). With the clip-on fan off, the keyboard is responding normally and I was able to work on my project for few hours. iMac is still hot. AC came on at a programmed time. Next day, I went back to my project for a couple hours before I turned on the clip-on fan. After one minute from turning on the fan, the keyboard stopped working! I always check to make sure the USB is firmly plugged in. *sigh* I turned off the clip-on fan. Reboot the iMac, all is working normally. I removed the clip-on fan from the stand and am completely puzzled by the fact somehow the clip-on fan is causing the keyboard to the iMac to be not functioning!