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The steps in this article may help with the symptom you're encountering:
When using a USB optical or Flash drive to install Windows, disconnect all other USB devices from the computer except for the USB Windows install media, keyboard, and mouse.
Follow these steps if you encounter this issue.
- Press and hold the power button to shutdown the computer.
- Remove all USB devices except for the Windows installation media, keyboard, and mouse.
- Turn your computer on again and immediately hold down the Option key until you see a screen from which you can choose your startup drive.
- Select the Windows install media to start the Windows installation process.
If you have a MacBook Pro (13-inch, Late 2012) computer, additional steps are required to successfully install Windows 8. See this article for details.