OneNote 2010 has a feature by which you can enable the automatic backup of a notebook. This can be done using the OneNote options menu.
Click the File menu –> Options link.
In OneNote options window, click the Save & Backup tab.
To turn on the automatic backup of notebook, mark the check box labelled as Automatically back up my notebook at the following time interval. You can specify the time interval using the drop down available with list of time intervals.
After making necessary changes, click OK button to save the changes.