Some clarifications on Xbox One behavior as it pertains to game updates, standby, and external hard drives:
If an Xbox is set to 'Instant On' mode and it has settings configured to keep the console OS and games/apps updated, the Xbox will check for updates during an overnight maintenance window when it is in standby (adjusted to local time) and automatically download/apply those updates. If an Xbox customer launches a game that has a content update that is required that hasn't yet been installed, the customer will receive a prompt that the game needs an update and the customer can then decide if they want to take a foreground content update or play the game offline. These foreground content updates are most commonly encountered if the Xbox hasn't yet had an overnight maintenance window in standby prior to a game update becoming required.
If an Xbox has games/apps installed on an external hard drive that have an available update during the maintenance window, the drive will power up to enable downloading and applying of the update. As others have mentioned, there is a setting to turn off external drives while in standby that is enabled by default. This setting is used to control external hard drives so that they will enter into a low power state while the console is in standby for power saving purposes. With this setting enabled, external hard drives should power up when called by the Xbox to enable content updates while the console is in standby. With that said, there may be some external hard drives that don't work properly with the power management commands being used, so this setting can be disabled to work around interop issues with those hard drives by keeping them powered up while the Xbox is in standby. Net-net, content on external hard drives should be updated when the Xbox is in standby with this setting enabled, barring any hard drive firmware interop issues.