This may occur on a MacBook Pro (13-inch, Early 2011) when the USB hard drive is attached to the front USB port, which is the port closer to the trackpad. Use the rear USB port—the port closer to the MagSafe adapter—to avoid this issue.
MacBook Pro (13-inch, Early 2011): Trackpad becomes inoperable after ejecting USB hard drive
When you use the Safe Removal menu in Windows to eject a USB hard drive attached to your MacBook Pro (13-inch, Early 2011) running Boot Camp 4.0 (which shipped with Mac OS X Lion), the trackpad may eject as well, causing it to become inoperable.
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