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SQL Error 5171 occurs when SQL Server fails to recognize an MDF file as a valid primary database file. This usually results from file corruption, version incompatibility, or missing database components. To fix it manually, open SQL Server Management Studio (SSMS), check the database status, set it online, and modify file settings using the ALTER DATABASE command. Then, stop the SQL service, copy the MDF/LDF files, and reattach them. If the issue persists or files are severely corrupted, use a premierl datarecovery.help SQL Database Recovery Tool. This tool repairs damaged MDF/NDF files, restores all objects (tables, views, triggers), and saves data safely.