hFile := CreateFile(PChar('\\.\PhysicalDrive' + IntToStr(No)), GENERIC_READ, FILE_SHARE_READ or FILE_SHARE_WRITE, nil, OPEN_EXISTING, 0, 0);
 if hFile = INVALID_HANDLE_VALUE then begin
   ShowMessage(SysErrorMessage(GetLastError));
   Exit;
 end;

 result:= DeviceIoControl(hFile, IOCTL_DISK_GET_DRIVE_GEOMETRY, nil, 0, @Geometry, SizeOf(TDiskGeometry), bytesreturned, nil)
   and (bytesreturned = SizeOf(TDiskGeometry));

 TDiskGeometry = packed record // http://msdn.microsoft.com/en-us/library/windows/desktop/aa363972%28v=vs.85%29.aspx
   Cylinders: TLargeInteger;   // eiee?anoai oeeeia?ia
   MediaType: DWORD;           // oei iineoaey
   TracksPerCylinder: DWORD;   // ai?i?ae ia oeeeia?a
   SectorsPerTrack: DWORD;     // naeoi?ia ia ai?i?ea
   BytesPerSector: DWORD;      // aaeo a naeoi?a
 end;

   memo1.Lines.Add('TotalBytes: '+ IntToStr(
     drive.Geometry.Cylinders * drive.Geometry.TracksPerCylinder *
     drive.Geometry.SectorsPerTrack * drive.Geometry.BytesPerSector
   ) );

Windows.GetDiskFreeSpaceEx('h:', FreeBytesAvailableToCaller, TotalNumberOfBytes, @TotalNumberOfFreeBytes);

memo1.Lines.Add('TotalBytes: '+ Format('%.0n',
     [drive.Geometry.Cylinders * drive.Geometry.TracksPerCylinder *
     drive.Geometry.SectorsPerTrack * drive.Geometry.BytesPerSector *1.0])
   );