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I would like to connect an external hard drive to the Raspberry Pi. This hard drive must be powered through USB and comes with a cable that has two ends (to ensure sufficient power). The cable is similar (not identical) to this one:

enter image description here

If I plug both ends into the Raspberry Pi, the hard drive shows symptoms of not getting sufficient power.

Is it safe to plug one end into an USB charger, and the other one (the one carrying data) into the Raspberry Pi? If I do this, do I risk damaging either the hard drive or the Raspberry Pi? Normally both ends would be connected to two different ports of the same computer.

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It is generally not possible to power a HD from the Pi, which has only limited current (depending on what else is connected), due to the polyfuse.

Connecting to a USB PS will attempt to backpower the Pi. I recommend a powered hub, but even in this case some will backpower.

It is not a good idea to connect a backpowering hub at the same time as a PSU.

You can easily check whether a hub backpowers by connecting without a PSU. If the red light comes on it MAY work - depends on the "USB charger". Many do not provide a sufficiently regulated voltage.

If you want more discussion search this Forum for "backpower".

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    Thank you, this is all useful information (I did not know about powered hubs). But it does not answer my question about the Y cable. Is using this cable the same as using a powered hub? – Szabolcs Jun 17 '14 at 20:16
  • Probably. There is no standard for Y cables (which are not in accordance with the standard). Most just have the +5V connected through. Give it a try (without the Pi PSU). You will run out of USB ports for a keyboard & mouse. – Milliways Jun 17 '14 at 23:45
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Based on this discussion, I thought backfeeding wasn't an issue on more recent (i.e. any Rev 1 or after boards)?

With that in mind, I don't see any problems with running the hard drive with that Y-Cable and your Pi with a separate power supply. It'd likely be easier with a powered hub since there'd be less cables.

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Power external hard drive though USB

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    That will not work on an A/B pi (this question predates the +/2 models), esp. a drive that requires two inputs for power, as shown. – goldilocks Dec 7 '15 at 13:16

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