copyFrom是不是不能拷贝很小的文件？？？

我的代码如下
if (SearchRec.Size <> GetLocalFileSize(LocalDIR+SearchRec.Name))  then
begin
  showmessage('文件的大小不一样！');
  FromFile:=TFileStream.Create(ServerDIR+SearchRec.Name,fmOpenRead or fmShareDenyWrite);
  try
    ToFile:=TFileStream.Create(LocalDIR+SearchRec.Name,fmOpenWrite);
    try
      ToFile.CopyFrom(FromFile,0);
    finally
      ToFile.Free;
    end
  finally
    FromFile.Free;
  end;
end;这么做对于只有几个字节的文件能识别大小不一样，但不会拷贝！  是不是copyFrom(FromFile,0)的那个0设置错误了？还是ToFile:=TFileStream.Create(LocalDIR+SearchRec.Name,fmOpenWrite)的fmOpenWrite错误了？请高手指点！