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I have a bad hard drive which could not be recognized by windows. If windows can recognize it, it could just be pure luck. There are better chance of it being recognize by plugging it at the usb back of the machine, rather than the front one. More interestingly, I have a usb cable which fork itself at one side ( i.e it has 2 heads at one side ). I can plug both heads to the machine and connect the portable hard drive on the other end of the usb cable? Can I get more voltage by doing this?

I am guessing the front usb port doesn't have enough power. I am wondering if I buy a new portable usb hard drive, would it have easy read by plugging at the front usb port?

I am not sure if I screw up the jumper connecting the front usb ports, but it is okay for other device such as usb stick and mouse.

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No, it's not common. Are you saying the sometimes Windows recognizes the external hard drive and at other times (on the same computer) it does not? –  martineau Aug 28 '12 at 1:52

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USB hard drives that have no separate power supply demand a lot of current from the USB port. Very often on a computer some ports are capable of providing more current than others. It may be that this hard drive is "on the edge" of drawing too much power, so it only works on the ports than can supply the most current.

If this is the case, the best solution (other than a new separately-powered drive) is probably a separately-powered USB hub.

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I've never seen such thing. They have something to provide power? –  lamwaiman1988 Aug 28 '12 at 3:39
    
@lamwaiman1988 -- You mean a separately-powered hub? They're quite common -- I've got one sitting on my desk here, another on the computer in the next room, and one on my desk at work. –  Daniel R Hicks Aug 28 '12 at 12:27
    
@lamwaiman1988 -- But another thing to check for is whether your disk drive has an "optional" power supply input. –  Daniel R Hicks Aug 28 '12 at 12:28
    
It has a DC input plug, but I have no idea have to give it a DC input. –  lamwaiman1988 Aug 28 '12 at 13:25
    
@lamwaiman1988 -- Look at the area around the plug. There should be a voltage embossed in the case and a symbol that looks like a "C" with a spot in it. You need to find an AC power adapter that has a plug that mates with the input socket, puts out the same voltage (make sure it's DC, not AC, for the output), and has the + and - voltages arranged to match the image (usually indicated by a similar image on the adapter). If the disk drive isn't too weird you'll find something in the pile of old AC adapters you have. –  Daniel R Hicks Aug 28 '12 at 19:25

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