function GetSerialNum(FDrive:String) :String;
var
  Serial : DWord;
  DirLen, Flags : DWord;
  DLabel : Array[0..11] of Char;
begin
  try
    Serial := 0;
    GetVolumeInformation(PChar(FDrive + ':\'), dLabel, 12, @Serial,
    DirLen, Flags, nil, 0);
    Result := IntToHex(Serial, 8);
  except
    Result := '00000000';
  end;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  // Lendo o serial do volume do Disco C.
  ShowMessage(GetSerialNum('C'));
 
  // Lendo o serial do volume do Disco D.
  ShowMessage(GetSerialNum('D'));
 
  // Caso não consiga ler a Unidade é retornado zeros
  ShowMessage(GetSerialNum('J'));
end;