如何检测A驱动器是否准备好？

Question

unit Unit1;interfaceuses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs,comobj, StdCtrls;type
  TForm1 = class(TForm)
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;var
  Form1: TForm1;implementation{$R *.dfm}procedure TForm1.Button1Click(Sender: TObject);
var x: OleVariant;
begin
x:=CreateOleObject('Scripting.FileSystemObject');
//-1:有盘
//0:无盘
MessageDlg(x.Drives.Item['a'].IsReady, mtInformation,[mbOk], 0);
end;
end.