Onenote - Reducing Storage Footprint on PC
You may modify some settings of OneNote 2016 to reduce disk space usage.
By default there are two separate backup copies, so every notebook will need triple the amount of space. This can be changed in File > Options > Save & Backup, under "Number of backup copies to keep". The default value of 2 can be changed to 1, reducing the space by one third.
All OneNote data is by default kept on the system drive. If you have an additional disk or partition, you may direct OneNote to use it instead.
For every currently opened notebook, there is another copy created on
the local cache in a special cache folder.
By default this is
This may be changed in File > Options > Save & Backup, under
"Cache file location settings", by clicking the "Modify…" button.
Note: Moving the cache to a removable drive or network location is not recommended.
A review of Microsoft's help files did not find a configuration to reduce notebook file sizes by only fetching notes and attached files as needed, but did show OneNote stores PDF files as images which can cause file sizes to balloon. If you save PDF files a lot, you may consider just saving their content instead.
For your use case of saving recordings, I was unable to find any way to reduce sampling rate or bandwidth to reduce the size of the WMA files which will be stuck into the cache, so it's time to make a use case to your CFO for a second drive or larger system drive (NVMe preferred, SATA SSD if not practical) in your PC.
I managed to free up 8gb of storage space.
Building on the solution offered by harrymc I first changed the location of the backup folder off of the local drive and onto a network folder. I then went back into the settings to performed a full backup to the new location:
File > Options > Save & Backup > Back Up All Notebooks Now
Depending on the size of your notebooks this may take quite a while.
Once the notebooks were backed up, closed each of the open notebooks. I then went to the Onenote folder on the hard drive and deleted the cache and back up folders (making a copy first just in case).
Finally I reopened the notebooks.
When the files were repopulated locally what I was left with was a single folder taking up 600mb. This was in place of two folders that were holding almost 11gb of data. A vast improvement.