cannot update Drupal core with drush Why can't Drush find the Drupal core module?  

root@hostname:/var/www/html/sites/example.org# drush pm-update core

...
Wysiwyg (wysiwyg)                        7.x-2.2         7.x-2.2         Up to date
core                                     Unknown         Unknown         Specified project not found


So then I tried:

root@hostname:/var/www/html/sites/example.org# drush -r /var/www/html -l example.org pm-update core
...
Wysiwyg (wysiwyg)                        7.x-2.2         7.x-2.2         Up to date
core                                     Unknown         Unknown         Specified project not found    


I have used both these commands in the past to update other Drupal modules and it has always worked, until I have to update the Drupal core module.  Can Drush not handle this or is this a bug?
To update core on its own try

drush pm-update drupal


A plain

drush pm-update


will update all modules, and then core.

Note that it doesn't backup .htaccess, robots.txt, or any other files that you may have modified.  I have gotten in the habit of doing a full drush archive-dump before core updates, and/or using Backup and Migrate in combination with having everything under git control.