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  1. #1
    Star Lounger
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    External Drives Forgotten

    I have used multiple external hard drives and secure digital cards on Windows XP, Vista and now 7.

    However, on the Win 7 laptop from time to time I will plug in one of the ext hard drives and it doesn't show up. I know how to find it and get a drive letter assigned but does anyone have any idea why this is happening.

    I have two ext hard drives and it has happened to both of them. Doesn't happen very often, thankfully.

  2. #2
    3 Star Lounger
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    Mar 2010
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    May have to do with USB power supply capability. USB 2.0 specifies nominal 5V, 500mA.
    Older USB flash drive, and USB hard drive (laptop sizes) maybe be at the max 500mA limit. When getting older, may draw more current, causing overload and USB outlet then self limits for protection. This causes the USB outlet 5V supply drops below 4.5V, at which point digital logic may malfunction. Then the PC/laptop may not detect the insertion of the USB device.
    Long thin USB cable, may create too much voltage drop to lower than 4.5V at the device side, may also cause insertion failure.
    Solutions:
    1. Try another USB outlet. Not all outlet has the same power capability. The 500mA current limit is not hard and fast. It depends on the IC chip used. Some can supply as much as 600mA to 650mA.
    2. Copper trace, and wire harness, leading to the USB outlet inside a PC/laptop maybe of different length. The shorter harness has lower voltage drop across its wires. Try another USB outlet helps.
    3. Use a shorter and better USB cable (with thicker wire gauge wires).
    4. If you use a USB hub, use a self-powered hub than a non-powered one.
    5. If using a non-powered hub, keep the total cable length as short as possible.
    6. If the USB hard drive has option for external power supply (a connector for external power), then use external power for the USB drive.

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