I found this very interesting forum yesterday, while searching for a way to jump a PSU, to work without the motherboard. I found the answer, then I spent the rest of the night, browsing the forum. I have an idea, but my knowledge of electricity, is very limited. I recently bought an old iMac G3, to tinker with. I have a 160 Gb sata HDD, connected with USB, powered with a PSU, from a dead Dell PC. I want to have the external PSU switched on, using the PS_ON# (green wire) to the COM (black wire), with a USB from the iMac, with a 5 volt relay. I'm looking for any advice, before I do this. I have a little understanding on how relays work (from automotive), but advice about resistors, if any are needed, please be very specific. I did a search, and didn't see any instructable that has been written on this. If this works out, I might try.