Symptoms
Assume that you use SQL Server 2017 or 2019. When you try to create a differential backup on a secondary replica, you receive a message that indicates that the command is running and may succeed:
BACKUP DATABASE WITH DIFFERENTIAL successfully processed {X} pages in {Y} seconds
Differential backups are not supported for secondary replicas. After you apply this fix, the correct error message is displayed:
Msg 3059, Level 16, State 1, Line LineNumber
This BACKUP or RESTORE command is not supported on a database mirror or secondary replica.
Msg 3013, Level 16, State 1, Line LineNumber
BACKUP DATABASE is terminating abnormally.
Note Currently, SQL Server does not allow differential backup on a secondary replica because it can’t modify the differential bitmap page. This is because the database is read-only. Additionally, copy-only transaction log backups are not allowed on a secondary replica. To maintain backup consistency, this hotfix prevents running a copy-only differential backup on a secondary replica.
Status
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
Resolution
This issue is fixed in the following cumulative update for SQL Server:
About cumulative updates for SQL Server:
Each new cumulative update for SQL Server contains all the hotfixes and all the security fixes that were included with the previous cumulative update. Check out the latest cumulative updates for SQL Server:
References
Learn about the terminology that Microsoft uses to describe software updates.
