OneNote's storage model is not the same as Word's or Excel's, and that trips people up. A Word document is one file. A OneNote notebook is a folder, and the things inside it are sections, backups and index files that only make full sense together. When a notebook stops working, knowing which of those files matters saves an enormous amount of wasted effort.

The mental model: notebook → section → page

OneNote nests three things:

  • A notebook is a folder on disk.
  • A section is one .one file inside that folder. Sections are the tabs across the top of the app.
  • A page lives inside a section. Pages are not separate files — they are records inside the section file.

So a notebook folder called Chemistry containing Lectures.one, Lab notes.one and Revision.one is a notebook with three sections. Every page you ever typed in the Lectures tab is inside Lectures.one.

.one — the section file. This is the one that matters

A .one file holds the pages of one section: your titles, your typed text, your outlines, and references to the images, ink and attachments you put on those pages.

Internally it is not a document in the way you might expect. It is a revision store: a structured binary container that keeps objects and successive revisions of them, so OneNote can sync incrementally and let several people edit at once without shipping the whole file each time. That design is why a .one is often larger than the text inside it suggests, and why a partially damaged one can still contain perfectly readable content — the older revisions are frequently still in there.

Two practical consequences:

  • It is not a zip. Renaming it to .zip and unzipping it does not work. Archive tools will simply report that it is not an archive.
  • It is documented. Microsoft published the format specifications, which is why independent readers for .one can exist at all. The bytes are not a mystery; they are just involved.

There are also two different shapes a modern .one can take: the classic desktop revision store, and the OneDrive-format package used by sections that sync to the cloud. They look identical from File Explorer. Any tool you use on them needs to handle both.

.onetoc2 — the table of contents. Structure, not notes

Every notebook folder contains a file called Open Notebook.onetoc2 or similar. Its job is to record which sections belong to the notebook, what order the tabs go in, and which section groups exist.

It contains no page text whatsoever. This is the single most useful fact in this article, because people regularly spend an afternoon trying to recover a .onetoc2 in the belief that it is the notebook. It is the contents page, not the book.

If you lose it, you have not lost notes — you have lost the ordering, and OneNote can rebuild a notebook from the .one files it finds. If you are archiving, keep it anyway: it is tiny, and it preserves the structure so a future you can reopen the notebook properly rather than as a pile of loose sections.

The 2 on the end is a version marker; older notebooks carry a plain .onetoc instead.

.onepkg — a packaged notebook

A .onepkg is a OneNote package: a whole notebook, sections and structure together, bundled into one file for backup or for handing to someone else. It is built on the Microsoft Cabinet (CAB) archive format, so unlike a .one, it genuinely is an archive — general-purpose tools that understand CAB can usually list and extract what is inside it, which turns out to be the .one section files you were after all along.

Worth knowing: the current Microsoft 365 OneNote does not create .onepkg files and does not import them directly. If someone hands you one, extracting the sections out of it and dealing with them individually is the practical path.

.onebak and the backup folder

OneNote takes automatic backups of your sections. On Windows these land by default under %LOCALAPPDATA%\Microsoft\OneNote\16.0\Backup, in a folder per notebook, as ordinary .one files. Older installs used 15.0, 14.0 or 12.0 in place of 16.0.

This folder is the most under-used recovery resource in OneNote. Before you reach for any tool, look in it. We list the full set of paths worth checking, on Windows and Mac, in where are my OneNote files stored.

How to tell an old file from a new one

There have been three broad generations of the section format, and which one you have decides what can open it:

  • OneNote 2003 format. The original. The modern desktop app does not support it. Moving one forward historically meant opening it in OneNote 2007 or 2010 first — a chain that is difficult to assemble now.
  • OneNote 2007 format. The modern desktop app can open and view these, but editing requires an upgrade to the newer format, offered as a Convert to 2010-2016 action in the notebook's properties. Note that the conversion is one-way: OneNote 2007 cannot read files in the newer format.
  • The 2010-2016 format. What current desktop OneNote uses, and what most files you meet will be.

You usually discover which generation you have by trying to open the file and reading the message you get back. The most common one has its own guide: “this section was originally created in an older version of OneNote”.

The rule that saves people: upgrading a file's format is a one-way door. Before you click any Convert button, copy the original somewhere safe. If the conversion goes badly you still have the file you started with, and if it goes well you have lost nothing but a few megabytes.

What about password-protected sections?

OneNote can lock an individual section with a password, and when it does, that section's content is genuinely encrypted inside the .one. A reader can tell you the protection is there. Reading the content requires the password — there is no side door, and we would not build one if there were.

Where a recovery tool fits

Knowing the format is often enough to solve the problem yourself: find the .one files, ignore the .onetoc2, check the backup folder, open them in the free OneNote desktop app for Windows. Our companion guide on opening a .one without OneNote walks that through step by step, free routes first.

Where a tool earns its place is the leftover case: a section file that no available OneNote will open, on a machine where installing one is not an option, when what you need is the typed text rather than a fully reconstructed notebook. OneNote Rescue reads both shapes of .one and parses .onetoc2 as well — and, being honest about it, says so plainly when a table-of-contents file carries no page text rather than pretending to recover something from it.

Honest scope. OneNote Rescue recovers page and section titles and note text, to plain text, a readable HTML notebook and a JSON report. From a OneDrive-format section it also writes the embedded images out as real files next to the notebook — along with any other embedded file whose type it recognises by its file signature — each with its SHA-256 recorded in the report. From a classic desktop revision store, embedded files are noted in the report but not extracted in this pass. Ink and handwriting strokes are noted, not converted to text, and any object it cannot identify is listed as present and not extracted rather than guessed into a file. Rich formatting and the exact page hierarchy are simplified into readable text. Nothing is ever decrypted or bypassed: password-protection markers are flagged in the classic revision store, and are not detected in the OneDrive-format container in this pass.

Supported operating systems: Windows 10 and 11 (64-bit), and macOS 10.15 or later. One purchase covers both, so you can recover the files where they already are — no copying them to another machine first.