Error: STC failure occurred mid transfer - Too small destination volume



This article explains the cause and solution of the error: STC failure occurred mid transfer - Too small destination volume.


  • Datto SIRIS
  • Datto ALTO


This error happens when disks have been recently re-sized, and the sparse image does not have enough space.

Figure 1: The error message


1. Ensure the latest versions of ShadowProtect and ShadowSnap are installed.  If a different version is installed, see Installing and Updating the ShadowSnap Agent for the download process.

2. This step depends on how backups were originally configured.

  • If the backups are unencrypted, and agent-based (either ShadowSnap or Datto Windows Agent), force a differential merge. If successful, run a new incremental backup on the agent that performed the diff merge. If the differential merge does not solve the issue, proceed to step 3.
  • If the backups are encrypted or agentless, proceed straight to step 3. 

3. Delete the sparse image from the web interface of the device.

    • Go to the web interface for the device.
    • Under the Protect tab, select Configure Agent Settings.
    • Select Advanced Settings followed by Volume Level Backup Control.
    • Now, delete the volume that was recently re-sized. There will be a warning: click OK to continue.

4. Delete the agent's full sparse image (i.e: all the volumes). In the web interface, select Advanced Options for the agent, and select Destroy Live Dataset. This will delete the full image of the agent, but have the same functionality with the old snapshots and the old live dataset.

5. Take a new backup. This will be a full base image and will reflect the size of the current dataset.

If this issue persists, contact Datto Technical Support for further assistance.

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