// add a reference to the System.Management assembly and
// import the System.Management namespace at the top in your "using" statement.
// Then in a method, or on a button click:

ManagementObjectSearcher theSearcher = new ManagementObjectSearcher("SELECT * FROM Win32_DiskDrive WHERE InterfaceType='USB'");
foreach (ManagementObject currentObject in theSearcher.Get())
{
   ManagementObject theSerialNumberObjectQuery = new ManagementObject("Win32_PhysicalMedia.Tag='" + currentObject["DeviceID"] + "'");
   MessageBox.Show(theSerialNumberObjectQuery["SerialNumber"].ToString());
}

کار دیگه ای لازم نیست

تست کردم مشکلی نداشت و راحت شناخت

Dll را اضافه کردید ؟