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I'm using a USB Y-Cable with my Pi400 to power and use an old WD 2TB MyPassport Usb3.0 HDD. I'm using a separate 5v-1a PSU so the HDD doesn't have brown-outs. So far I've been using using the cable and brown-outs have disappeared and it boots up just fine.

I read that back-powering issues with the Pi4 can consist of failed boots and nothing else, but there doesn't seem to be a clear answer or verdict.

Is it safe to use, how can the Pi400 be damaged by this configuration?

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EDIT:

I'm rewording the question to be more concise.

Can you please leave this question and delete the other one? thanks

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    Welcome -- but please do not ask the same question repeatedly because you do not like the answer(s) you get. if you want to reword your question, reword the original.
    – goldilocks
    Commented Apr 2, 2023 at 16:22
  • thanks, but unfortunately my other post was not concise enough and the merit of the question not covered in the answer. Could you please close the other one and leave this one active instead, if that’s an option?
    – luckyluca
    Commented Apr 2, 2023 at 19:19
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    I'm sorry, but no. You are still free to edit the original, as long as it is not too radical a change. Currently I think the feedback you have gotten is accurate and appropriate, and this does not requre anyone spending more time on it. You are never going to get a guarantee this is 100% risk free. If you want to dive deeper, generalize it and use our bigger sibling, Electrical Engineering. WRT "back-powering issues with the Pi4", this is not really back powering as you are still using a primary supply.
    – goldilocks
    Commented Apr 2, 2023 at 19:34
  • @goldilocks - It's also been posted on another site, with similar answers. Including mine, where I suggest that this is dangerous and there is another way of doing it. But the OP appeared to want a guarantee that no damage would occur. Commented Apr 3, 2023 at 0:41
  • I actually don't think it is necessarily dangerous in the sense that inevitably something bad will happen, but at a minimum it does create a risk because the protection on the power jack is being by-passed. Since the second supply is really just for the HDD, the best idea might be to open the cable up and cut the USB power line between the power and the jack that attaches to the Pi.
    – goldilocks
    Commented Apr 3, 2023 at 13:23

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The behavior of a USB host being powered through a downstream port is not described in the USB specification. Practically, this means that the electrical engineers can implement whatever design they see fit, ranging from an absolutely safe schematic which can be back-powered indefinitely with no ill side-effects, to a borderline-unsafe design which blows up within seconds.

Real devices always fall somewhere between these two extremes, usually closer to the safe one. However, since there is no certification tests for such out-of-spec behavior, nobody (including designers themselves) really knows for sure. In particular, there's no guarantee that it's repeatable from one board to the next, and that it's stable over time.

I suppose in your case the convenience of the solution that you have already tested, the apparently low risk of damage, and the low cost of the hardware involved make this solution acceptable.

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