Since upgrading to 11.10 from 10.10 (fresh install) my laptop (Thinkpad X60) no longer goes into standby if the power is removed after the lid has been closed.
Is there any way I can go about re-enabling this?
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I use kubuntu (KDE), so you'll have to "translate" this into your interface.
Since the ubuntu underneath is essentially the same, you'll be able to do the same things, but your menus may look different.
Under KDE, I open System Settings. In the Hardware section there's a Power Management option.
In that, there's a Global Settings Tab with an item "When AC power is unplugged". It shows which Power Profile the system will use when this event occurs. Mine says "Powersave".
There's another Tab for Power Profiles. On that tab, I click on "Powersave" and it brings up a panel with all the actions that the system will take when this profile is activated including what to do if the notebook lid is closed.
While you're there, check out the rest of your Power management settings and set them to your preferences.
Personally, I don't like sleep and hibernation, so I set all of them to do nothing or shutdown.