In a MDB file, the data are stored as continuous pages with a fixed size. The first page, called database definition page, contains the most important definitions of the database.
If the page structure in the MDB file is damaged, for example, several bytes in the head of the file are lost permanently, Access will not be able to recognize the pages in the file and will report this error.
If the database definition page or other important data is damaged, then Access cannot recognize the database format and will report the error, too.
In a word, as long as Microsoft Access cannot recognize the MDB file as a valid Access database, it will report this error.
You can try our product DataNumen Access Repair to repair the MDB file and solve this error.