Un-insulated Ground wire in USB cable
Non-insulated wire inside the USB cable is SHIELD. It should be isolated from BLACK ground wire in the cable, and connected only to metal shrouds of USB connectors. On a system level, the shields should be routed separately, and connected to signal/main ground at a single point via decoupling R-C network (0.1uF  || 330R recommended), usually at the power supply feed point and Earth ground of the system (if any).