string result = String.Empty;
             try
             {
                 ManagementObjectSearcher searcher = new ManagementObjectSearcher("SELECT * FROM Win32_DiskDrive");
                 foreach (ManagementObject d in searcher.Get())
                 {
                     result = d["SerialNumber"].ToString().Trim();
                     //break;
                 }
             }
             catch (Exception ex)
             {
                 result = ex.Message;
             }

  PSerialNumberParams = ^TSerialNumberParams;
  TSerialNumberParams = record
   wInfoLevel:word;
   dwDiskSerialNumber:longint;
   caLabel:array[0..10] of char;
   baFileSystem:array[0..7] of char;
  end;
function GetDriveSerialNumber(wDrive: word): LongInt;
var
	snp: TSerialNumberParams;
begin
	snp.dwDiskSerialNumber := 0;
	if ReadDriveSNParam(wDrive, @snp)
	then Result := snp.dwDiskSerialNumber
	else Result := -1;
end;

function ReadDriveSNParam(wDrive: word; psnp: PSerialNumberParams): boolean; assembler;
asm
  push ds
  mov  bx, wDrive
  mov  al, 00h
  mov  ah, 69h
  lds  dx, psnp   //<=вот тут ошибка Operand size mismatch
  int  21h
  jnc  @no_error    	{CF SET if error}
  xor	 ax,ax		{set false}
  jmp	 @exit
  @no_error:
  mov	ax, 1			{set true}
  @exit:
  pop	ds
end;