FileVault and update needed at the login screen: How to solve
Sometimes, when FileVault or FileVault2 are just enabled on the Mac (specially with Lion OS X 10.7.x or Mountain Lion OS X 10.8.x), you may see at the login screen a unknown user account named “update needed”. Additionally, in the login preference pane you can’t choice the “automatic login” option.

Well, the problem can be solved easily but it is time-consuming.
Follow the steps below:
- Log in into the Mac as administrator.
- Backup the Mac with Time Machine (optional but recommended)
- Launch System Preferences selecting System Preferences from the Apple menu.
- Click the Security & Privacy preference pane.
- Click the FileVault tab.
- Unlock the pane clicking on the padlock (administrator password required)
- Disable FileVault and wait (…about 3hrs with a 250GB no SSD HD)
- Restart the Mac (optional but recommended)
- Go again at the FileVault tab.
- Enable FileVault and wait (…about 7hrs for me!!!)
- Restart the Mac. The unknown user account will disappear. Now you can also choice the automatic login.
Good Luck.