TROUBLESHOOTING DIFFERENTIAL BACKUP FAILURES
When running a differential database backup, SQL Backup Master may report a failure with the following SQL Server error:
Failed to create backup: Cannot perform a differential backup for database "XYZ", because a current database backup does not exist.
A differential backup records only the changes made since the most recent full backup, so SQL Server requires a valid full backup – the base of the backup chain – to exist before a differential can be taken. This error means the backup chain has been invalidated. There are three common causes.
Cause 1 – VSS (Volume Shadow Copy Service) backups
Third-party backup solutions that use the Windows Volume Shadow Copy Service create database snapshots that invalidate the SQL Server backup chain, interfering with SQL Backup Master operations. The component responsible is the SQL Server VSS Writer, which operates via a Windows system service called the SQL Writer Service.
To prevent VSS-based products from breaking the backup chain:
- Press Windows + R, type services.msc, and press Enter.
- Find and double-click the SQL Server VSS Writer service.
- Click the Stop button and change the startup type to Manual.
- Click OK.
- Trigger a full database backup in SQL Backup Master to re-establish the backup chain. Subsequent differential backups will then succeed.
Note: SQL Backup Master includes a Backup History report (available from the Reporting tab) that helps identify problematic snapshot backups – look for entries whose Is Snapshot field shows True. Those entries were created by a VSS-based backup product rather than by SQL Backup Master.
Cause 2 – Changing the recovery model
When a SQL Server database's recovery model is changed, the backup chain is disrupted. Manually trigger a new full backup in SQL Backup Master to reinstate the backup chain and enable future differential backups to run successfully.
Cause 3 – SQL Server 2005 bug
If you're running a legacy SQL Server 2005 instance, Microsoft Knowledge Base article #921106 documents a bug in that release which can cause this issue, and a hotfix is available. This cause only applies to SQL Server 2005 – if you're on any later release, focus on the first two causes above.
Related resources
If differential backups continue to fail after reviewing the solutions above, please contact our support team and we'll be happy to help.