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Different devices look for or detect different signals besides just power on usb connections. My Creative MP3 player could only charge from certain usb ports and only using certain ac adapters that provided some signal that the player was seeking. It was not just a matter of having the right voltage.
My Blackberry will occasionally throw up a prompt/message that there was insufficient power to charge it - has happened with both cigarette lighter sockets and usb ports on laptops.
Some devices will not work properly with usb hubs whether they are separately powered or not and sometimes the usb ports on the front of a computer and those on the back will provide different results. USB is not, well, always usb, I gather.
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