Here are some common causes and solutions for users facing an "Unknown Device" problem. This post is a follow-up to Why is my USB device not detected or comes up as "Unknown Device"?.
1) The USB device is "hung".
The USB device may have encountered an internal error and simply stopped functioning. The device will have a code 43 in Device Manager. Removing all power from the device (e.g. unplugging and re-plugging the USB cable and also the power cable if it has one) will reset the device and allow it to enumerate successfully.
2) Worn or faulty USB connectors and cables.
Over time USB cables and connectors can fail. Try plugging it into a different USB port, and also using a different USB cable if it is removable. The device will have a code 43 error in Device Manager.
3) No driver is installed, and the USB device does not report a device description string.
In this case Windows was unable to find a driver for the device, and the device did not report a device description string. The device will have a code 28 in Device Manager. To resolve this problem you will need to find a compatible driver for the device, typically by contacting the device’s manufacturer.
4) There is insufficient power to start the device.
If the device is downstream of a bus-powered hub, the device can enumerate successfully but may not be able to start due to insufficient power. This a bus-powered hub cannot supply more than 100mA to a port, whereas a self-powered hub can supply 500mA. The device will have a code 28 in Device Manager. To resolve this problem, plug-in the hub’s power supply (if it has one), move the device to a root hub port, or move the device to another hub that is self-powered.
5) The upstream hub has failed or is "hung".
If the device is connected to an external hub, try moving the device to a different hub or a root port. If the device enumerate successfully, try power-cycling the hub or removing/replacing it. Device will have a code 28 in Device Manager.
6) The USB device has failed.
It is possible that the USB device itself has suffered a hardware failure and is no longer functional. You will need to replace the device. Device will have a code 43 in Device Manager.