Tip of the Moment 

Planning to do a lot of audio recording in OneNote? It's worth it to invest in a decent external microphone. This is the one I have.

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How Do I Open My Notebooks in OneNote 2010?

In OneNote 2010 where are my notebooks and how do I open them?

The default location for notebooks in OneNote 2010 is in the My Documents\OneNote Notebooks folder.  There you should see all of your notebooks listed as folders.  Within those folders you'll see the OneNote sections listed as .ONE files.

You can open the notebook by double-clicking on the "Open Notebook" file that you see within the notebook folder in Windows Explorer or in OneNote from the File | Open | Open Notebook dialog box.

TIP: You can also open a notebook by simply opening any section (.one file) in the notebook.  Opening a section opens the whole notebook.

It's possible that your notebooks won't be located in the (My) Documents\OneNote folder if you've changed the default storage location or if you manually saved a notebook somewhere else.

To check the default storage location go to File | Options | Save & Backup and the box at the top will show you your storage locations.
File storage in OneNote 2010

If you manually saved the notebook somewhere else (like a flash drive or network location) things get a little trickier.  If you don't remember where you saved it then your best bet is probably to do a Windows search across all possible locations for "*.ONE" files.