unit UMain;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
  StdCtrls, FileCtrl;

type
  TForm1 = class(TForm)
    DriveComboBox1: TDriveComboBox;
    procedure DriveComboBox1Change(Sender: TObject);
  private
    { Private declarations }
    function DiskInDrive(Drive: Char): Boolean;
  public
    { Public declarations }
  end;

const
  DefaultDrive = 'C:';

var
  Form1: TForm1;

implementation

{$R *.DFM}

{ TForm1 }

uses StrUtils;

function TForm1.DiskInDrive(Drive: Char): Boolean;
var
  ErrorMode: Word;
begin
  if Drive in ['a'..'z'] then
    Dec(Drive, $20);
    //make the drive uppercase
  if not (Drive in ['A'..'Z']) then
  //make sure it's a letter and, if it's not
    raise  //...raise an exception
      EConvertError.Create(Drive + ' is not a valid drive ID');

  ErrorMode := SetErrorMode(SEM_FAILCRITICALERRORS);
  //turn off critical errors

  try
    if DiskSize(Ord(Drive) - $40) = - 1 then
    //DiskSize(1) = a, DiskSize(2) = b etc..., DiskSize(- 1) = not a
                //valid drive
      Result := False else
      Result := True;
    finally
      ErrorMode := SetErrorMode(ErrorMode);
      //reset original error mode
  end;
end;

procedure TForm1.DriveComboBox1Change(Sender: TObject);
var
  CurrentDrive: String;
begin
  if DriveComboBox1.Drive = '' then
    CurrentDrive := DefaultDrive else
    CurrentDrive := DriveComboBox1.Drive + ':';

  if not (DiskInDrive(CurrentDrive[1])) then
  begin
    MessageDlg('There is no disk in drive ' + ChangeCase(CurrentDrive),
                mtError, [mbOK], 0);
    DriveComboBox1.Drive := 'C';
  end;
end;

end.