I'd like to use an external usb3 2Tb WD MyPassport 5200rpm HDD with my Raspberry Pi400 for a large Retropie project.

I think having it running powered from the Pi400 alone is not giving enough power to it: I once noticed some ext4 errors on reboot, which perhaps is caused by the lack of power.

I was thinking of using a USB 3.0 Y-cable (which I already own) to supply enough power to the external HDD: https://www.amazon.co.uk/InLine%C2%AE-USB-Y-Cable-Micro-Black/dp/B00NQHZBF8

Y-cable enter image description here Would it be ok to use it and to plug the power-only usb-A connector to a 5v 1a power adapter?

I've been told that it might back-power the USB3.0 port of the Pi400. I've so far tested it and it seems to be running OK, and brown-outs have disappeared. But I stopped for fear of damaging the Pi400.

Can you please kindly let me know what damage could this create?

What I tried so far: I run the external WD mypassport 2TB 5200rpm HDD directly with the Pi400 and noticed some exr4 errors on reboot. Which makes me think it's a brown-out.

I run the Y-Cable plugged to a 5V-1A adapter and to the HDD and into the Pi400 and haven't had any problems.

I also tested a usb3.0 powered hub that worked well for a week and it is now causing some brown-out symptoms, so I'd like to return it and explore using the Y-Cable instead.

1 Answer 1


That's what those cables are for. Some drives come with them. However, it depends on how that cable has been wired and how the power on the USB connector on the Pi has been designed. This is because the disk will simultaneously draw power from both the plug and the pi.

Some devices, such as high-speed external disk drives, require more than 500 mA of current[89] and therefore may have power issues if powered from just one USB 2.0 port: erratic function, failure to function, or overloading/damaging the port.

Such devices may come with an external power source or a Y-shaped cable that has two USB connectors (one for power and data, the other for power only) to plug into a computer. With such a cable, a device can draw power from two USB ports simultaneously.However, USB compliance specification states that "use of a 'Y' cable (a cable with two A-plugs) is prohibited on any USB peripheral", meaning that "if a USB peripheral requires more power than allowed by the USB specification to which it is designed, then it must be self-powered."

From How is a Usb Y cable designed?

Whenever you want to connect the +5V line to all three terminals is up to you. If you do, USB spec guarantees that voltage on D+ and D- will never rise above that level, causing damage. On the other end, you'll short two +5V regulators together (the one in your TV and your external adapter), which will create parasite currents between them. It's hard to tell which option is best, in fact, USB spec prohibits Y-cables straight away because they can't be implemented properly.

From Wiring of a USB Y-Connector

It seems clear to me that there is a risk that it may be damaging. That said, I have had disk drives that came with Y cables and required power from two USB sources. I also have a screen that does just this. And before, this question, I had never thought about any damage.

But I have gone destroyed 2 motherboards and 2 USB hubs. I now know why. My advice is that don't do it this way.

Use a powered USB hub such as this and use a straight cable.

enter image description here

  • This is good general info but it doesn’t cover my question specifically to what damage would backpower cause to the Pi400 if any. Also powered usb hubs can also back power the Pi400. I can’t see any evidence online of people reporting any issue other than inability to boot in such case. Using the Y cable in my circumstance solved brown issues and there are no issues with booting. I would like to establish what other damage there could be of any to the Pi400.
    – luckyluca
    Apr 1, 2023 at 17:18
  • That is unknown. It depends on the design of 3 things. My last section is a suggestion on the safe way to go. Apr 2, 2023 at 4:27
  • Whilst I appreciate the answer, it is generic and it does not cover the merit of the question or answers the original question, which is: What damage could having up to 5v-1a entering one of the usb-a 3.0 ports of a Pi400, if any. Please note using the y-cable this way, has resolved brown-outs and is not causing reboot issues, which is the symptom others reported online.
    – luckyluca
    Apr 2, 2023 at 14:13
  • 1
    "That's what those cables are for." -> I don't think so. They are intended for plugging the two male jacks into two ports on the same hub/machine/power source, to get around USB 2.0 power limitations (500 ma per jack). The OP is instead plugging them into two different power sources.
    – goldilocks
    Apr 2, 2023 at 16:29

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