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I have 2 USB hard drives that combined take too much power to run through the pi USB port.

I have several 5V sources laying around that provide plenty of current for all of the hard drives and raspberry pi combined.

Could I wire up the USB any of these two ways to power the hard drives via the external source?:

schematic

simulate this circuit – Schematic created using CircuitLab

schematic

simulate this circuit

In the first case, the Hard drive power is in parallel with the USB power. In the second case, the pi power is disconnected and the external power is connected directly to the HDD.

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At best this would be unnecessary, and at worst destructive in some way.

Yes of course theoretically you could do this though... USB 2.0 ports can supply 500mA, so I assume you need 1A for the two drives. Allow 1.2A for the pi.

I'm not an electrical engineer, but I believe that if you wired a power supply to power two usb-powered drives and a raspberry pi in series, you would just end up with a power supply that didn't supply enough current to power any of them.

Remember Ohm's law, and at least simulate what you're trying to do with a circuit simulator - remember that circuit simulators aren't the real world though.

I enjoy DIY projects, but I just power the drives separately. My pi is connected to an ancient external drive that runs off an AC adapter, and they're both very happy.

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