I have a slightly different scenario here: I created a new user from the command line as I broke my original user (and that is another problem altogether) and while having fixed most permission problems using vigr, found that I couldn't shut the machine down or mount USB drives. Following the solution from christianwolf above, I looked at /etc/lightdm/lightdm.conf and found that autologin was set to the old username. Commenting that out and rebooting has given me access to the USB drive and I'm yet to test rebooting.
-- You received this bug notification because you are a member of Desktop Packages, which is subscribed to lightdm in Ubuntu. https://bugs.launchpad.net/bugs/851055 Title: [Oneiric] 'Not Authorized' error given when mounting device Status in “lightdm” package in Ubuntu: Fix Released Status in “lightdm” source package in Oneiric: Fix Released Bug description: Oneiric Ocelot with latest update. Nautilus, Disk Utility are unable to mount/format/create partitions on USB drives. User is presented with 'Not Authorized' error. ProblemType: Bug DistroRelease: Ubuntu 11.10 Package: gvfs 1.9.5-0ubuntu1 ProcVersionSignature: Ubuntu 3.0.0-11.18-generic 3.0.4 Uname: Linux 3.0.0-11-generic x86_64 ApportVersion: 1.23-0ubuntu1 Architecture: amd64 Date: Thu Sep 15 18:07:58 2011 EcryptfsInUse: Yes HotplugNewDevices: /dev/sdb /dev/sdb1 /dev/sdb3 /dev/sdb2 HotplugNewMounts: InstallationMedia: Ubuntu 11.10 "Oneiric Ocelot" - Beta amd64 (20110901) SourcePackage: gvfs Symptom: storage Title: Does not detect hotplugged storage device UpgradeStatus: No upgrade log present (probably fresh install) To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/lightdm/+bug/851055/+subscriptions -- Mailing list: https://launchpad.net/~desktop-packages Post to : [email protected] Unsubscribe : https://launchpad.net/~desktop-packages More help : https://help.launchpad.net/ListHelp

