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I have a dell optiplex 960. If I plug in 3 usb devices (keyboard, mouse, printer) all to 3 usb ports next to each other (there is a row of 4 and a row of 3 on the back of the computer), It will not power on and the power button will flash every second and the computer powers on for about 1/4 of a second for every second. However if I plug in 2 to the bottom row and 1 to the top row, it turns on fine. Is this because maybe one row cannot handle the amount of current to all those usb devices or is this a bigger issue?

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It could be over current, but it shouldn't be. Did you plug at totally different device into the port in question like a memory stick, and does it boot then? It is possible that the port is damaged and has an internal short circuit. Although the keyboard and mouse both require power from the PC, the printer should not draw any power from the pc since it has its own source of power.

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I have seen USB devices that allowed power to feed back into their feed wires. This could prevent the computer from booting if it was plugged into certain ports. (It also was destructive, the ports would soon die.)

(I finally caught on to what was going on one day when the power supply was unplugged and yet the motherboard had a I'm-powered-up light on it. It finally went away when I unplugged the offending hub.)

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I had a similar experience. I solved it temporarily by removing the power to the USB hubs. Powering the USB devices themselves (the ones that could) was OK. But I will have a problem the day when I need to connect a device that draws too much current and can not be powered independently. – Peter Mortensen Jul 30 '15 at 19:12

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