For IDE harddisk you can use S.M.A.R.T. API function for
extracting serial number. For SCSI it does not work.

program ScsiSN;

// PURPOSE: Simple console application that display SCSI harddisk serial number

{$APPTYPE CONSOLE}

uses
  Windows, SysUtils;

//-------------------------------------------------------------
function GetDeviceHandle( sDeviceName : String ) : THandle;
begin
  Result := CreateFile( PChar('\\.\'+sDeviceName),
    GENERIC_READ or GENERIC_WRITE,
    FILE_SHARE_READ or FILE_SHARE_WRITE,
    nil, OPEN_EXISTING, 0, 0 )
end;

//-------------------------------------------------------------
function ScsiHddSerialNumber( DeviceHandle : THandle ) : String;
{$ALIGN ON}
type
  TScsiPassThrough = record
    Length             : Word;
    ScsiStatus         : Byte;
    PathId             : Byte;
    TargetId           : Byte;
    Lun                : Byte;
    CdbLength          : Byte;
    SenseInfoLength    : Byte;
    DataIn             : Byte;
    DataTransferLength : ULONG;
    TimeOutValue       : ULONG;
    DataBufferOffset   : DWORD;
    SenseInfoOffset    : ULONG;
    Cdb                : Array[0..15] of Byte;
  end;
  TScsiPassThroughWithBuffers = record
    spt : TScsiPassThrough;
    bSenseBuf : Array[0..31] of Byte;
    bDataBuf : Array[0..191] of Byte;
  end;
{ALIGN OFF}
var dwReturned : DWORD;
    len : DWORD;
    Buffer : Array[ 0..SizeOf(TScsiPassThroughWithBuffers)+
                       SizeOf(TScsiPassThrough)-1] of Byte;
    sptwb : TScsiPassThroughWithBuffers absolute Buffer;
begin
  Result := '';
  FillChar(Buffer,SizeOf(Buffer),#0);
  with sptwb.spt do
  begin
    Length   := SizeOf(TScsiPassThrough);
    CdbLength := 6; // CDB6GENERIC_LENGTH
    SenseInfoLength := 24;
    DataIn := 1; // SCSI_IOCTL_DATA_IN
    DataTransferLength := 192;
    TimeOutValue := 2;
    DataBufferOffset := PChar(@sptwb.bDataBuf)-PChar(@sptwb);
    SenseInfoOffset := PChar(@sptwb.bSenseBuf)-PChar(@sptwb);
    Cdb[0] := $12; // OperationCode := SCSIOP_INQUIRY;
    Cdb[1] := $01; // Flags := CDB_INQUIRY_EVPD;  Vital product data
    Cdb[2] := $80; // PageCode            Unit serial number
    Cdb[4] := 192; // AllocationLength
  end;
  len := sptwb.spt.DataBufferOffset+sptwb.spt.DataTransferLength;
  if DeviceIoControl( DeviceHandle, $0004d004, @sptwb, SizeOf(TScsiPassThrough), @sptwb, len, dwReturned, nil )
    and ((PChar(@sptwb.bDataBuf)+1)^=#$80)
  then
   SetString( Result, PChar(@sptwb.bDataBuf)+4,
     Ord((PChar(@sptwb.bDataBuf)+3)^) );
end;


//=============================================================
var
  hDevice : THandle = 0;
  sSerNum, sDeviceName : String;

begin
  sDeviceName := ParamStr(1);
  if sDeviceName='' then
  begin
    WriteLn;
    WriteLn('Display SCSI-2 device serial number.');
    WriteLn;
    WriteLn('Using:');
    WriteLn;
    if Win32Platform=VER_PLATFORM_WIN32_NT then // Windows NT/2000
      WriteLn('  ScsiSN PhysicalDrive0')
    else
      WriteLn('  ScsiSN C:');
    WriteLn('  ScsiSN Cdrom0');
    WriteLn('  ScsiSN Tape0');
    WriteLn;
    Exit;
  end;
  hDevice := GetDeviceHandle(sDeviceName);
  if hDevice=INVALID_HANDLE_VALUE then
   WriteLn('Error on GetDeviceHandle: ',SysErrorMessage(GetLastError))
  else
    try
      sSerNum := ScsiHddSerialNumber(hDevice);
      if sSerNum='' then
        WriteLn('Error on DeviceIoControl: ',
          SysErrorMessageGetLastError))
      else
        WriteLn('Device '+sDeviceName
          +' serial number = "',sSerNum,'"');
    finally
      CloseHandle(hDevice);
    end;
end.

// For more information about SCSI commands:
// ftp://ftp.t10.org/t10/drafts/scsi-1/
// ftp://ftp.t10.org/t10/drafts/spc/
// ftp://ftp.t10.org/t10/drafts/spc2/