[marikris]

Most may not power-up right off the bat.. they may power-on for 1/2 a second, then stop, if there is no load on the +5V.. this load can be 10-ohm,(10W) to 33-Ohm (5W).. Most ATA hard drives draw about 6-7W, dependent on the maker... (as long as nothing is connected to the IDE socket, powering up/down a hard drive will not delete data.) The switching circuitry will compensate on the load, maintaining the correct voltage.

A test I do, make the short to the PWR-on to GND as you describe. Plug the supply in to the wall. The cooling fan will power-up, If it needs a load, the fan will just go on for 1/2 a second, then spin-down. Unplug the supply from the wall.. Within 2 seconds, as the filter caps bleed-down, the cooling fan will spin-up again for only 1/2 a second. This is the switching circuit simply noticing no use load, and simply shutting down