How to fix Error 4320 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 4320

Why This Error Happens

Error 4320 occurs when SQL Server detects that a data file is being restored out of sequence, typically because an earlier differential or transaction log backup in the restore chain was skipped or applied incorrectly. This commonly happens when a NORECOVERY restore sequence is broken, a backup file is corrupted, or the Log Sequence Number (LSN) chain between the full backup and subsequent files does not match. It can also surface when a file was previously restored using WITH RECOVERY prematurely, locking the database out of further restore operations.

Step-by-Step Resolution Guide

Step 1: Verify the Backup Chain Integrity

  • Run RESTORE HEADERONLY FROM DISK = 'C:\Backups\YourFile.bak' to inspect the LSN values (FirstLSN, LastLSN, CheckpointLSN).
  • Confirm that the DatabaseBackupLSN of your differential or log backup matches the CheckpointLSN of the full backup used as the base.
  • Cross-reference timestamps in msdb.dbo.backupset to ensure no backup files are missing from the sequence.

Step 2: Restart the Restore Sequence with Correct Syntax

  • Drop the partially restored database if it is stuck in a “Restoring” state: DROP DATABASE YourDB;
  • Restore the full backup fresh using the NORECOVERY clause:
    RESTORE DATABASE YourDB FROM DISK = 'C:\Backups\Full.bak' WITH NORECOVERY;
  • Apply each differential and log backup strictly in chronological order, keeping NORECOVERY on every file except the final one.

Step 3: Check File System and Storage Health

  • Navigate to the backup directory (e.g., D:\SQLBackups\) and verify the .bak or .trn files are not flagged as “corrupted” or zero-byte by running CHECKSUM during the backup process in the future.
  • Use RESTORE VERIFYONLY FROM DISK = 'C:\Backups\YourFile.bak' to validate the specific file causing the 4320 error.
  • If using Windows Server 2025 Storage Spaces or SMB shares, check Event Viewer under Windows Logs > System for disk I/O errors that may have truncated the backup file during the write process.

Step 4: Finalize the Recovery

  • Once all transaction logs are applied in order, issue the final recovery command to bring the database online:
    RESTORE DATABASE YourDB WITH RECOVERY;
  • Ensure no backup files in the sequence were skipped between the first and last LSN numbers identified in Step 1.

Alternative or Verification Steps

  • Verify the Fix: Query SELECT state_desc FROM sys.databases WHERE name = 'YourDB'; to confirm the status reads ONLINE rather than RESTORING.
  • Alternative Solution: If the backup chain is unrecoverable due to a missing log file, you may need to accept data loss up to the last valid full/differential backup by terminating the recovery with RESTORE DATABASE YourDB WITH RECOVERY immediately after the last successfully applied file, bypassing the corrupted log segment.