I was wondering what it had to do with the Raspberry Pi considering the hostname, and was secretly hoping this could be a way to use a bigger power supply to power in a Raspberry Pi and also supply 12V to standard 3.5" HDD. That would simplify the wiring for those who wants to make a NAS.
You can use an ATX PSU for that. You may need a load resistor depending on the model if the HDD is low power.
Connect 5v to RPi (with a fuse and potentially a zener diode), connect 12v to HDD via standard molex, job done.