Drupal site that points to a non-drupal .html page I'd like to configure my Drupal 6's site's front page to be "foo/index.htm", a hand-coded html file. But when I put that relative URL into Home › Administer › Site configuration › Default Front Page, it complains "The path 'foo/index.htm' is either invalid or you do not have access to it." It's a unix filesystem and everything is world-read, and the directories are world-execute. What could be the problem here?
The issue is the path in question here must be a valid Drupal path, not a Unix path/filename.  I'd suggest investigating hook_menu() as a first step of learning about Drupal paths, callbacks and so forth.