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I just hooked up an external Kingston SATA drive with a SATA-USB adaptor to my Pi 2B. I powered the disk from a separate USB hub, the Pi had its power from a common micro-USB PSU.

Then something a bit scary happened: I unplugged the Pi's PSU, and... it kept running. Dumbfounded, I unplugged the network cable, because that was hooked to a PoE switch, although if the Pi could be PoE-fed I would have noticed by now.

But it kept on running, and did so until I unplugged the external HD. In other words, the Pi was receiving power over the data cable from the HD.

It doesn't boot when I plug in the HD, but if I jumpstart it with the PSU I can plug it out and it just keeps running on the HD's power.

The main question here is: is this normal, or is there a chance that the Pi has a defect?

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I do not think it is recommended.

Some USB hubs can backfeed power. I don't know if they are meant to but it does happen.

It was not uncommon for the early Pi models to be backpowered via USB.

I think the Pi2 and Pi3 have altered circuity and can not be booted by backpowering. However once booted they will switch in the USB circuitry and can then be backpowered.

  • I do not think it is recommended is really the take-away message here. – Dmitry Grigoryev Dec 12 '16 at 11:59
  • Okay, I'll emphasise that. – joan Dec 12 '16 at 12:03
  • I just plugged the power for the HD to my laptop, and it's still backfeeding, so It doesn't seem to be because of the USB hub. Anyways, it's good to hear that it's a known pehonmenon. – UncleBob Dec 12 '16 at 12:12
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I'm going to provide a few details on why it is not recommended to do this.

  1. You completely bypass the protection circuit. the RPi has several components on the power socket, such as the polyfuse F1 and what appears to be a polarity protection Q3. When you backfeed from the USB, those components are essentially excluded from the circuit. Should your RPi fail for some reason, it will draw excessive current until your hub or HDD or laptop cut it off. That is, if those devices happen to have their own protection.

  2. You're stressing components with excessive current. Everything going to the USB socket (capacitors, PCB traces, power switches) are designed to support the current of a single USB load, 500mA max. Now, the whole current consumed by the RPi (and other USB devices you may have) goes through this path. Now, good electronics are designed with a fair amount of margin, which is why you don't see smoke right off. But beware: most components have reduced lifespan when stressed with excessive current, so your RPi will live less happy years if you constantly power it in this way (and when it will die, GOTO #1).

It's not like you or your board are in immediate danger, but I wouldn't keep back-feeding the RPi without a good reason do to so.

  • While I agree that you bypass the polyfuse, I wince when I read your other comments (as I am sure any Electrical Engineer would)! Just to take 1 point, the capacitors are totally unaffected by any current flowing, unless it has an unacceptable ripple. You can quite safely draw 1.2A from USB. See Raspberry Pi Power Limitations – Milliways Dec 13 '16 at 0:47

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