SIRIS, ALTO, and NAS: Error: Backup failed due to a process timeout on the protected system (error code -121)



When you try to start a backup for a system protected by the ShadowSnap agent, you receive the error message, "Backup failed due to a process timeout on the protected system."

In legacy versions of the Datto IRIS environment, this error message was "Snapshot returned error code -121."


  • Datto SIRIS
  • Datto ALTO
  • Datto NAS 


This issue can be caused by latency or lag in the responsiveness of the LAN during the transfer of a backup job to the Datto device. Other possible causes include:

  • The agent is experiencing networking issues when trying to write the backup image to the Datto device.
  • The agent is experiencing a timeout when attempting to write data.
  • Something is preventing large data transmission across the network. Some antivirus programs will lock access to files if data is being constantly manipulated.


To minimize transient errors, update the ShadowProtect and ShadowSnap agents to the latest version (click to download) and reboot the target machine.

Network Troubleshooting

This error can be caused by a network communications issue between the Datto appliance and the protected machine. Make sure that you have read the Datto Networking Requirements. To test and solve, try these steps:

  • Can the Datto appliance be accessed from the machine? Test ping and UNC paths to the device.
  • Create a NAS share on the Datto device and try writing data to that share from the protected system.
  • Add exceptions to any antivirus or firewall software for the ShadowSnap application on ports 25566 and port 139, and the C:\Program Files (x86)\StorageCraft folder.
  • If you are protecting machines across multiple subnets, ensure that a static route is configured on the network or use an auxiliary NIC provided on the Datto SIRIS to communicate with this subnet.
  • Ensure that there is no firewall, proxy, or filter of any type is interrupting communication between the Datto SIRIS device and the ShadowSnap agent on TCP port 25566 of the protected system.
  • Try replacing the Ethernet cable that is plugged into the Datto SIRIS to help resolve any packet loss experienced as the result of a faulty cable.

Server Health / Issues with the Protected Machine

Follow these steps to check for and correct problems on the target system:

  • Review the Tuning Guide for StorageCraft Software on Servers (external link) article to improve server reliability.
  • A minimum of 20% free space per protected volume and 1 GB of RAM need to be available on the production machine. If resources fall below these thresholds, backups will fail.

     Volume Use



    0 – 70%


    Use is good.

    71 – 80%


    Use is outside optimal limits. Monitor the volume growth and look at cleaning up unnecessary files and moving non-essential data to other storage.

    81 – 100%


    Volume is becoming critical. Snapshots will start to fail. Disk queue lengths will increase. Create space.

    Figure 1: StorageCraft disk use best-practices.

  • Check for scheduled tasks that might conflict with the backup: Run schtasks on the Windows command line.
  • Review Microsoft Event Logs to determine if there are I/O issues present on any services and inspect the server's overall health. Compare the events around the time of the failed backup, and determine if there are conflicting services.
  • Look for VSS events. If you see VSS events 12293 or 12298, see the following article: VSS Errors 12293 and 12298: What does it all mean? (external link)
  • Look for VSNAPVSS events. If you see any, see the following article: Event Viewer Warnings for VSNAPVSS: How to decipher this message (external link)
  • Restart the Microsoft Volume Shadow Copy service. If you see the error message "Windows could not stop the Volume Shadow Copy service on Local Computer. Error 1053: The service did not respond to the start or control request in a timely fashion," ensure that the server's Page File size appears to be adequate, and reboot the server to clear the hung service.

If all the above steps fail, perform a chkdsk /r and disk defragmentation on the target machine.

Contacting Technical Support

If the issue persists, collect the ShadowProtect diagnostic logs and contact Datto Technical Support.

Additional Resources 

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