Existing site drops me back into the install routine
This seems like a permissions problem. If you are in Linux, use the "chmod" command to set the correct permissions for settings.phpYou may need to have superuser privileges to do this so see your site admin.

Often, the web-server runs as user "www-data" with group "www-data" so you may need to use the "chown" command (once again with superuser privileges) to change its ownership first.

Example: 

# sudo chown www-data:www-data settings.php

# sudo chmod 444 settings.php


You will have to adjust the permissions again before editing this file in the future.