Hi I just bought a nespi case and have an issue, If I have a USB hard drive plugged in using an external power source my power button does not turn off the PI as it is drawing power from the USB, is there a way to fix this without cutting the usb wires?
Most of the current Pi models (B+ and later), but not Zero, have a well engineered power circuit which prevents the Pi from being backpowered by the USB ports (which was a problem with earlier models).
However if power is removed and voltage is supplied by USB devices (which should NOT happen) the Pi may continue to run.
Unfortunately the USB portion of the circuitry is not published.
On the original Pi I used a custom cable with a Schottky diode in series with the 5V line on the USB to allow normal devices to be powered by the Pi, but prevent backpowering.