the database file will, probably, be on system partition which is not accessible via USB.
If you want experiment, you may install OTFM file manager for Onyx. You'll find the link in this Onyx forum. OTFM has access to system partition. If you have SQLite datadase experience, you might be able to fix the (possibly) wrong value for LastPageRead.
The file will have probably .db extension, but one never knows... *.sketch files are also SQLite db files.
There are also utilities for console (via USB) and/or wifi access. Would be more convenient do the search using PC to using OTFM file manager. Even OTFM is able to "allow" access to system partition from PC. The feature is called Big Brother, but I do not understand how to use it...
Some/all of the tools I mentioned are referenced in Wiki.
SW for M92 Wiki