Ok so I just had a weird problem with my Mac Pro 2009 that I spent 6hrs trying to fix, but am still confused. What happened was that I wanted to feed a SATA data cable up under the PSU and into the DVD drive cavity (which I have empty) as I'm putting a SSD drive up there and connecting to the data port of a Apricorn PCI card. Anyway, took everything out including PSU, put it all back in, went to turn the Mac on and it lit up, fans turned on, but no boot! Things like usb mouse / keyboard don't turn on (no lights). Pressing the DIAG button and everything looks ok (PSU PWROK - green, 5V STBY - amber, EFI DONE - green) but GPU OK is flashing green??? What does that mean? I'm sure that has something to do with why it wasn't booting right up. Anyway, I took everything out again. Checked all cables. Did a basic external multimeter test of the PSU using a paperclip to turn it on, and the 12V rails seem to be fine, so I think the PSU must be ok. Put everything back in. Turned it on and this time it did start up! Pressing the DIAG button now showed PSU PWROK - green, 5V STBY - amber, EFI DONE - green (GPU OK - no light). But only for a maybe 20 sec, then it restarted and went back to how it was before - GPU OK is flashing green, fans spinning but no boot. Re-plugging the power cord, and turning it on again didn't change anything.... However... now when I just plugged the power cord back into the PSU I must have done it slower or something as it made a crackle sound (kind of like when you plug a power cord into a wall outlet sometimes), and it started up properly!! It's been on for a few hours now and has been running solid. So what happened to it?