MB: Gigabyte GA-EP45-UD3P. CPU: Intel E8400 3.0 GHz. All was running 24/7 as a server for 3.5 years.
I was going to back up my RAID 5 data to an extra disk, so I shut down the PC and opwned the case and plugged the extra drive in. Thereafter it would not boot.
Sympoms when trying to power on: power kicks on for 0.5 second, then PSU clcks and power goes off to all components. 2 seconds later the cycle repeats and it will do this forever. I tried unplugging everything and removing all MB cards (kept memory). I kept the 20(?) pin power coard and the CPU cord on the MB. Same symptoms upon retry. I then unplugged the MB CPU power cable and MB power lights stay lit on the MB. Next, I plugged the CPU power cord back on the MB but removed the CPU. Success again. Thus all appears to be good as long as the CPU is not in the socket.
Do I have a problem with the CPU, or MB? I already have a new PSU because I thought that was the problem. Now the MB and the CPU will be the same cost if I have to make another parts replacement, so I need some analysis to help me determine how to proceed.