“I can’t open accdb files in Access 2007 after they’ve been opened in 2010,” was posted on Access TechNet Forum. Its because “Access 2010 brings many new features (Data Macros, Calculated Columns, etc.) that Access 2007 doesn’t understand. If Access 2007 open a database created/modified in 2010, it’ll ignore these new features and might even cause data loss or corrupt business logic. For e.g., a data macro validation rule that’s forced in 2010 but ignored in 2007, this can cause data inconsistency. To prevent that from happening, Microsoft implemented the feature to block certain databases to open in Access 2007 after it’s modified in 2010, or open it as read-only in 2007. Access 2007 cannot open a database built or modified in Access 2010 if it uses: new sort orders, Access 2010 new encryption, or your application has been published to SharePoint,” explained Access 2010 blog. For more info refer Features that are not available in Access 2007 on MSDN.