function SerialHDD(const DriveLetter: Char): string;
var
  NotUsed:     DWORD;
  VolumeFlags: DWORD;
  VolumeInfo:  array of Char;
  VolumeSerialNumber: DWORD;
begin
  GetVolumeInformation(PChar(DriveLetter + ':'),
    nil, SizeOf(VolumeInfo), @VolumeSerialNumber, NotUsed,
    VolumeFlags, nil, 0);
  Result := Format('Label = %s   VolSer = %8.8X',
    [VolumeInfo, VolumeSerialNumber])
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  ShowMessage(SerialHDD('c'));
end;