type
  TAofExt = array[1..1024] of Extended;

  TestTt = class(TTestCase)
  public
  procedure SetUp; override;
  procedure TearDown; override;
  published
  procedure Test;
  end;

implementation

procedure TestTt.SetUp;
var
  f: File of TAofExt;
  r: Extended;
  ae: TAofExt;
  i,j: integer;
begin
  try
  try
    AssignFile(f, 'D:\myfile.bin'); // или на C:\Users\me\Desktop\
    Rewrite(f);
    for i:=i to 1024*1024 do
    begin
      for j:=1 to 1024 do 
      begin
        r:=Random;
        ae[i]:=r;
      end;
      Write(f, ae); // когда i=1025 сначала Access denied, следом I/O error 103.
    end;
  finally
    Close(f);
  end;
  except
    on E:Exception do
      //
  end;
end;

ae[i]:=r;