Drush and update.php checks directly against PDO if the pdo_xxx extension is loaded. Since autoslave is a "virtual" db driver, Drush and update.php will fail, because there exists no such driver as pdo_autoslave.
To "fix" this, use the master connection explicitely when drush or update.php is invoked.
<?php
// Bypass AutoSlave entirely when using update.php or drush
if (drupal_is_cli() || basename($_SERVER['PHP_SELF']) == 'update.php') {
$databases['default']['default'] = $databases['default']['master'];
}
?>