Drush pm-update needs a module? When trying to run `Drush pm-update' in a site i get the error 

Command pm-update needs the following modules installed/enabled to run: update.                              [error]
The drush command 'pm-update' could not be executed.


Oddly this command was recently working (on other sites in the same core installation), and further more I have no idea where to find or what is this 'update' module. Can anyone point me in the right direction?
It's in &lt;drupal_root&gt;/modules/update. If not, it means you damaged your core.

Try drush en update - it should re-enable it for you. After performing updates you are allowed to disable it again.