[ecfisa]

Hola danielmj.

Proba de este modo:

Código Delphi [-]
unit ...
{$WARN UNIT_PLATFORM OFF}

...

implementation

function MediaInDrive(Drive: Char): Boolean;
var
  ErrorMode: Word;
begin
  ErrorMode := SetErrorMode(SEM_FAILCRITICALERRORS);
  try
    Result := DiskSize(Ord(Upcase(Drive))-Ord('A')-1) = -1;
  finally
    SetErrorMode(ErrorMode);
  end;
end;


procedure TForm1.DriveComboBox1Change(Sender: TObject);
begin
   with (Sender as TDriveComboBox) do
    if not MediaInDrive(Drive) then
      raise Exception.Create('Inserte un disco en la unidad '+ UpCase(Drive) +':');
end;

...

Saludos