SIRIS, ALTO, and NAS: Fatal I/O error on .datto: #112 on write: there is not enough space on the disk



Backups fail with the error: "Fatal I/O error on .datto: #112 on write: there is not enough space on the disk."


  • Datto ALTO
  • Datto SIRIS


The error code -112 is an official Microsoft System Error code for "There is not enough space on the disk." 

  • This is commonly seen on machines which have protected volumes that have less than 20% free space.
  • The Datto device doesn't have enough space to accept the backup.
  • There may be corruption in transfer of the backup image across the network.


  • Rule out any processes that are taking up large amounts of resources. Datto recommends maintaining at least 30% free space per protected volume. A minimum of 20% free space per protected volume, and 1 GB of RAM, must be available at all times on the production machine. If resources fall below these thresholds, backups will fail. Figure 1 explains utilization levels in detail.
For Datto Windows Agent volume space requirements, Refer to the System Requirements section of this article.

 Volume Utilization



0 – 70%


Utilization is good.

71 – 80%


Utilization 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.

  • Exclude all Recovery or SRP partitions from the Advanced Options on the agent tab.
  • Check networking and other connections to the target machine.
  • This error may appear if multiple base images are running from the same protected machine.
  • Try to write a Shadow Copy of the volume as a test from the Shadow Copy or a Windows Backup to determine if the VSS writers can actively write a copy.
  • Have Datto technical support verify the device storage pool is healthy.
If none of these fix the issue, contact Datto Technical Support for further assistance.

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