You just need to close the huge notebook and then do File -> Open Notebook
for each notebook that you want to open. For example, if your folder
structure looks like:
OneNote Notebooks
- Notebooks I Copied
- - Math
- - History
- - Science
If you click on "Notebooks I Copied" as the notebook to open, Math, History,
and Science will be opened as section groups. So instead, you should open
Math, History, and Science separately.
In the event that you only copied the .one files and not the folder
structure, you can simply create a folder structure on disk. The basic idea
here is that there is no concept of a "notebook" on disk. Only .one files.
It's really quite simple. So whatever folder on disk the .one files are in,
that is your notebook and that is the folder you should open. If you open a
folder that contains other folders, those folders will be section groups.