Since getting my iPad 3 at launch, it has never backed itself up. It always displayed “Unknown” and the “Back Up” option was missing from the right-click menu. Today I finally found a forum post with the solution. I’m reposting it here in hopes that more people find it:
o At first, close iTunes.
o Then, open Terminal and type the following commands in:
defaults write com.apple.iTunes DeviceBackupsDisabled -bool false
defaults write com.apple.iTunes AutomaticDeviceBackupsDisabled -bool false