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I've got a USB 2.0 hard drive enclosure which came with a cable with 2 plugs on it. I know that this is done so that it can draw more power if necessary.

If the hard drive can't draw enough power with just one of the plugs, does the second plug have to attach to the same computer? Or can I get my power by attaching the second plug directly to a wall-wart?

(Specifically, I am planning to attach this drive to a Raspberry Pi, where power and USB space is limited)

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This should work fine. However, to be safe you should probably use a powered USB hub rather than a power adapter as typically used for charging a mobile device as some of them are designed to use non-standard voltage/amperage.

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In general, using a power adapter won't cause any problems. The USB spec is designed such that it negotiates the charging amperage: extremetech.com/computing/… –  cmorse May 13 '13 at 2:59
    
Usually, but many manufacturers ignore the USB spec when it comes to their power adapters, often to increase the wattage and reduce charging times. Unless you are sure of what the power adapter is doing, you are safer using a device that is USB specific. –  YLearn May 13 '13 at 3:02
    
That doesn't mean that the charger is going to force that amperage on the device. The device is in control of how much amperage it will draw. See the answers here for an explanation: android.stackexchange.com/questions/4494/… –  cmorse May 13 '13 at 3:29
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I've not tried it, but it should be ok. the device will only draw the power it needs off the second usb plug.

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