Show a maintenance page while moving the site I am going to move the finished drupal site from the sub-directory to root, because the files are quite large, it takes a while to do so, I want to put a html file there to show 'site maintenance in process, be back soon'. 

How can I do this while I move everything?
First put your site into the maintenance mode than ("Administer > Site configuration > Site maintenance"), and than do the following:

Create the template file for maintenance page as maintenance-page.tpl.php.

Tutorial for creating a template file for maintenance page: https://drupal.org/node/195435