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I want to connect an external hard drive to a Raspberry Pi via USB, but the hdd draws too much power. Thinking of isolating 5v+ and ground from the hdd's USB cable, connecting it to a power bank, and only linking data pins between the Raspberry Pi and the hdd was wondering if there would be any problems. While i'm not sure, apparently i might have to wire the ground for the power bank, hdd and the pi's usb together?