Drush error when attempting Drupal update: unable to copy/move
We can only guess at an answer. A likely scenario might be disk space, permissions, or some other conflict with Drush &amp; filesystem, but that's speculation. 

Run the same command with the --debug flag (drush up drupal -y --debug) to see a verbose dump of what Drush is doing and look for errors/warnings that occur as Drush is attempting a core update. Once you know the underlying filesystem issue you'll be able to resolve it.