The registry contains entries informing the system how to recognize and connect USB devices. If these entries become corrupt, change or are otherwise rendered unusable, USB 2.0 devices may display as unknown devices when connected. Microsoft's Fixit services can automatically repair the damaged portions of the registry. You can also use a third-party registry repair program such as Uniblue Registry Booster or Advanced Registry Optimizer.Malfunctioning Drivers
Missing, malfunctioning, outdated or virus-corrupted drivers may also cause connected USB 2.0 devices to register as unknown, even if the system is properly configured. To fix nonfunctional drivers, remove them. Type "Device Manager" into the Start menu search field and press "Enter." Double click on the USB device and navigate to the "Driver" tab. Click "Uninstall" to remove the driver. Reinstall the driver from the device's CD or the manufacturer's website. Use caution, however: never uninstall a driver until you have a way to reinstall a working copy.