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#11
Those PC fans are meant to work with 12 Volts. USB ports provide 5 Volts, so it won't turn as fast as it should. Then there is the current to consider. You may want to look at the fan to see if it has a label saying how much current it draws in Amps or milliAmps. You don't want to go over 500 milliAmps when drawing from a USB port or you'll damage it.
My advice: If you want a USB powered fan, buy one specifically designed for the purpose.