Fix for Time Machine after Mountain Lion upgrade

After upgrading from Lion to Mountain Lion on two machines, the Time Machine backup application announced that it couldn't find the Time Capsule backup - even though that volume could be mounted and inspected manually.  There's a simple fix...

• Go to System Preferences / Time Machine
• Turn Time Machine off
• Make a note of the settings (i.e. what the target disk is, and any excluded directories)
• Quit the System Preferences application
• Get to a command shell and execute:
      sudo rm /Library/Preferences/com.apple.TimeMachine.plist
• Go back to Go to System Preferences / Time Machine
• Select the target disk again
• Re-enter your settings (i.e. any excluded directories)

Your historic backups will be preserved (and visible if you choose to "Enter Time Machine").

Time Machine will find your backups (and tell you the oldest and latest backup dates, etc) after the next time you let Time Machine run.

As others have noted, Time Machine now supports encrypted backups.  So if you don't need your historic backups, then you might want to zap them and start again with encryption turned on.