Datto Windows Agent: Every Backup Runs as a Differential Merge



On DWA, every backup runs as a differential merge, even if it is supposed to be an incremental backup.


  • Datto Windows Agent


Check permissions on the protected volume root files.

The Datto Windows Agent must be able to create, delete, read, and modify three different files in the root of each protected volume: DattoCTRLDattoSNAP, and .datto.mif (the .datto.mif file is created during the backup of that specific volume, and then deleted when the backup of that volume is complete, so you will not normally see it unless a backup of that volume is in process).

Datto Windows Agent backup services run under the Local System user account by default, so this account must have (at least) Read/Write permissions on the root of each protected volume. Ideally, the Local System User account should have Full Control. If it does not have these permissions the agent will not be able to determine incremental change, and will always fail over to a diff merge.

If this happens, you will see errors like this in the agent logs (the 5 at the end is the error code for Access Denied):

Open snapshot current incremental bitmap file [driveletter]:\DattoSnap failed: 5


Open snapshot merged incremental bitmap file [driveletter]:\.datto.mif failed: 5

After the errors, the next line in the log will be:

SetupMergedIncrementals failed, forcing diffmerge.

Update Permissions

Change permissions on the root of the volumes noted in the errors above to allow the Local System account to have full control, or at least Read/Write permissions. Full control is ideal, as the device must be able to create and delete files in the root.

If a backup job is currently running for this agent as a differential merge, this backup must complete successfully before you can verify that the Datto Windows Agent has returned to running normal incremental backups as expected, and no errors appear in the logs.

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