I have a notebook that has a file path associated with my old backup HD. I no longer have that backup drive but onenote still lists it to try and sync it. Unfortunately I cannot delete it (path doesn't exist) and I cannot close it as it is simply greyed out. It's just in my list and there seems to be no way to close or get rid of it. Any ideas?

Hi,
If you are not afraid to use the registry editor then take a look at this key:
HKEY_CURRENT_USER/Software/Microsoft/Office/14.0/OneNote/OpenNotebooks

There you will see all the 'Open notebooks' on the right side of the editor. Hover over the various books and find the one that does not exist anymore. Delete that entry.

Be warned to create a backup of your registry first before tampering with it!

It does carry some risks but is definitely the best answer thus far. Thanks.