$ drush dl drupal
$ mv drupal-7.x/ drupal
$ cd drupal
$ cp ~/Sites/drupal/sites/default/default.settings.php ~/Sites/drupal/sites/default/settings.php
$ chmod -R a+w ~/Sites/drupal/sites/default
$ drush dl drupal
$ mv drupal-7.x/ drupal
$ cd drupal
$ cp ~/Sites/drupal/sites/default/default.settings.php ~/Sites/drupal/sites/default/settings.php
$ chmod -R a+w ~/Sites/drupal/sites/default