ManagementObjectSearcher theSearcher = new ManagementObjectSearcher("SELECT * FROM Win32_DiskDrive WHERE InterfaceType='USB'");
            foreach (ManagementObject currentObject in theSearcher.Get())
            {
                string str = currentObject["PNPDeviceID"].ToString();
                str = str.Substring(str.LastIndexOf('\\') + 1);
            }

SELECT * FROM Win32_DiskDrive WHERE InterfaceType='USB'