Synology NAS Error Code 66: How to Fix It

Medium 30-60 minutes Medium Severity Verified June 2026
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Error Code
66
Brand
Synology
Product Type
nas
Severity
Medium
DIY Difficulty
Medium
Estimated Fix Time
30-60 minutes
Synology NAS Error Code 66 indicates that an Active Backup for Business job has failed to complete successfully. This can happen due to network interruptions, insufficient storage space, permission issues, or problems with the target device being backed up. While this is a medium-severity issue, it means your data is not being protected as scheduled, so resolving it promptly is important.
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Tools You'll Need

How to Fix Error Code 66

  1. Check the Active Backup for Business Log

  2. Verify Network Connectivity to the Backup Target

    Only enable SSH temporarily for diagnostics. Disable it again when you are done to reduce your NAS's security exposure.

  3. Check Available Storage Space on the NAS

  4. Re-enter Credentials for the Backup Task

  5. Verify the Synology Active Backup Agent on the Target Device

  6. Check Firewall and Port Settings

  7. Update DSM and Active Backup for Business Package

    Back up your DSM configuration before performing a DSM update. Go to Control Panel > Update & Restore > Configuration Backup to export a config backup.

  8. Delete and Recreate the Backup Task

    Deleting a task will remove associated restore points unless you select 'Keep backup data' when prompted during deletion. Choose this option to preserve your existing backup history.
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When to Call a Professional

Contact Synology Support or a certified IT professional if: the error persists after completing all steps above; your NAS is part of a business-critical environment with Active Directory or domain-joined machines and you are unsure about credential or permission configurations; the DSM logs show hardware-related storage errors such as bad sectors or volume degradation alongside Error Code 66; or your backup protects regulated data (healthcare, finance, legal) where data loss compliance is a concern. You can reach Synology Support at Synology.com/support.

Frequently Asked Questions

What does Synology error code 66 mean?
Synology error code 66 means that an Active Backup for Business job failed to complete. This is typically caused by network connectivity problems, insufficient storage space on the NAS, incorrect login credentials for the target device, the backup agent not running on the source machine, or firewall rules blocking communication.
Will I lose my backup data if I delete the failed Active Backup task?
Not necessarily. When deleting a task in Active Backup for Business, Synology will ask whether you want to delete the associated backup data or keep it. Choose 'Keep backup data' to preserve your existing restore points. However, you will no longer be able to restore from those points once the task is deleted unless you recreate a task pointing to the same data location.
How do I check why my Synology Active Backup job failed?
Open DSM and go to Active Backup for Business > Task List. Click the failed task, then select 'Log' from the menu. The log will display a timestamped record of the job attempt with a specific error message explaining what went wrong, such as connection failure, authentication error, or out-of-space condition.
How much free space does Synology Active Backup need?
Active Backup for Business requires enough free space to store deduplicated backup data for all protected devices, plus overhead for new changes and retention versions. As a general guideline, Synology recommends your backup volume has at least 20-30% free space at all times. If your volume is above 80-85% capacity, you should either expand storage or reduce your retention policy to prevent backup failures.
Can Synology Active Backup error code 66 happen if the source PC is turned off?
Yes. If the target PC or server is powered off, asleep, or hibernating at the scheduled backup time, Active Backup for Business cannot connect to the agent and the job will fail with error code 66. To prevent this, configure your PC's power settings to disable sleep during scheduled backup windows, or use Wake-on-LAN if your NAS and network support it. You can also schedule backups during hours when the machine is reliably powered on.

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