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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?

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  • I realise it does not solve your problem but: This is the fault of the drive. It should not be supplying power back to the host. If you look around you will find various reports of people using poorly made powered hubs that do the same thing to a motherboard. So for future reference, don't buy that brand of drive!
    – goldilocks ♦
    Mar 9, 2018 at 21:26
  • It is a known feature of the PI that it can draw power from USB devices. and hard drives virtually all require a separate power supply for the PI due to needing more AMPs than the PI USB slots can provide it really is not about quality of the hub or the device it is simply how the PI is made to work
    – romx
    Mar 9, 2018 at 21:40
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    No. A USB device should not supply power to the host. That you can backpower the pi has nothing to do with that. superuser.com/a/764958/257378
    – goldilocks ♦
    Mar 9, 2018 at 22:17

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I think I have come up with the simplest solution and cut the USB power line going to the pi and use a new USB plug connector stopping power going back into the PI system and allowing shutdown. I posted this answer in case it helps anyone else facing the same issue

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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.

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  • Yeah I decided to order USB fittings to modify the lead so it does not supply power
    – romx
    Mar 10, 2018 at 22:05

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