My client machine was recently due for a rebuild however I needed to ensure that my OneNote Book was maintained as I had important screenshots dating back to early last year. By default, your OneNote books are backed up daily and can be found in C:\Users\%Username%\AppData\Local\Microsoft\OneNote\14.0\Backup. Ensure your OneNote Books are saved to a network location and that the backups are also set to a network location: