How to change the database drupal forms send to It's super easy using php and SQL but how do you change the default database drupal 6 saves posted data in the forms? To an external database?
I would try reading through How to connect to multiple databases within Drupal (notably, the Drupal 6 section towards the end.

Basically, in your settings.php file, convert the $db_url to an array, and add the secondary database:

e.g.

$db_url = 'mysql://drupal:drupal@localhost/drupal';


becomes

$db_url['default'] = 'mysql://drupal:drupal@localhost/drupal';
$db_url['db2'] = 'mysql://user:pass@localhost/db2';


Then create a submit handler that either compliments or overrides an existing form submit handler and use db_set_active to connect to the secondary database, save the data, and then switch back.

e.g.

function MYMODULE_MY_EXTRA_SUBMIT_HANDLER($form, &amp;$form_state){
  // Switch to db2  
  db_set_active('db2');

  // Do all your form processing and database transactions

  // Connect back to default Drupal database
  // *Note* If your primary db is keyed with "default", then you don't need to
  // pass an argument to db_set_active().
  db_set_active('default');
}