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When upgrading I always put the individual site in maintenance mode, take a backup of the DB with mysqldump, modify the symlinked htdocs directory and then execute update.php. But before I upgrade production I always test a dev version of the site because you never know. Once the update.php script is done I take the site out of maintenance mode but only after an initial view of the site. If the initial view doesn't look right I can restore the database and change the symlink.
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Hi Ernie thanks. Yep I had both server sites in maintenance mode but I did not do so with the home sites, I wanted to see if it crashed the sites. I too mysqldump routinely. Problem is, because the drush up corrupted the drupal 7.12 install there is no way to update.php, both were dead. My curiosity is that it had no effect on my home setup but destroyed the remote server drupal set up.
Google drupal 7.12 and find the drupal.org/node/1425104 - posts dated 2nd Feb 2012 posted by webchick discuss the 7.12 problems. I found this too late. Roger