“Unrecognized database format” — what it means and how to fix it

You double-click an .accdb or .mdb file and Access answers Unrecognized database format 'C:\…\file.accdb' (error 3343). The file is almost never “not a database”. It is a database written in a dialect of the format that your copy of Access does not read.

The four usual causes

1. An .accdb opened in Access 2003 or older. The .accdb format arrived with Access 2007. Anything earlier cannot read it at all, whatever the error says.
2. A newer data type inside the file. A database that uses the Large Number type (Access 2016 build 16.0.7812 and later) or Date/Time Extended (2020 and later) is refused by every earlier version with exactly this message, even though both are “.accdb”.
3. A damaged file. A copy interrupted mid-write, a network drop while the database was open, or a full disk. Access reads the header, finds it inconsistent, and reports the same error.
4. Not an Access file. A renamed export, a backup container, or the .laccdb/.ldb lock file that sits beside a database while it is open. Check the file size: a lock file is a few hundred bytes.

Fixes, in the order worth trying

1. Match the version. Open the file on a machine with a current Microsoft 365 Access. If it opens there, the file is fine and your copy is simply older than the file.
2. Compact & Repair a copy. Copy the file first. In Access, open a blank database, then Database Tools → Compact and Repair Database and pick the copy. This rebuilds the structure and rescues most interrupted-write damage.
3. Restore a backup. If Compact & Repair also fails, the file is genuinely damaged; a backup from before the incident is the only complete fix.
4. Just read the data. If the goal is the records rather than the application, you do not need a compatible Access at all. AccessLens reads the file format directly — every era from Access 97 to 2019, including Large Number columns — in a Chrome tab on your own machine, and exports any table to CSV, or the whole database to Excel or SQLite. It cannot repair a truly corrupt file (it reads the same bytes), but it is not affected by version mismatches.

Questions