Should be easy enough to grab 5V of the wiring harness for the HDDs. J8 is the place to go for your high power 12V or 5V needs. https://wiki.kobol.io/helios64/sata/#j8-pinout (there is even a picture with the wire harness).
The GPIO header is not the right place to get high current from .)
5v - 12v At your peril ! https://wiki.kobol.io/helios64/sata/#j8-pinout
12v Maybe you can possibly bridged the input power to J16 https://wiki.kobol.io/helios64/hardware/#connector-interface-list
3v - 5v but not 2.5a https://wiki.kobol.io/helios64/ gpio /# pinout
Helios64 is not intended for this. the best would be to make a space size of your power cable on back cover of the case and plug it into a power source provided for this purpose.
