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MARS agent error

Diana Piloya 185 Reputation points
2026-03-05T03:33:07.38+00:00

I continously keep getting this error when performing backup and havent got any successful one. what is hampering the process? User's image

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  1. Q&A Assist
    2026-03-05T03:38:23.8466667+00:00
    AI generated content. This question contains an answer created with AI from Q&A Assist. Learn more

    The screenshot shows repeated failures of a System State backup with the MARS (Microsoft Azure Recovery Services) agent. The message “encountered an unexpected failure while transferring data to Microsoft Azure Backup” is a generic transfer error, but the context shows that System State backups typically fail because of underlying VSS, disk, or configuration problems on the server rather than a pure network issue.

    The main factors that can be hampering the backup are:

    1. General MARS agent / environment prerequisites not met
      Before deeper debugging, verify the standard requirements for a healthy MARS backup:
      • MARS agent is updated to the latest version.
      • Stable network connectivity from this server to Azure.
      • MARS services are running; restart the MARS services or the server, then retry the backup.
      • 5–10% free space exists on the volume that hosts the scratch (cache) folder.
      • Antivirus or other processes are not scanning/locking the scratch folder or backup files.
      • OS has current updates, the machine clock/time zone is correct, .NET Framework 4.5.2 or later is installed, and TLS 1.2 is enabled.
    2. System State–specific issues (most common root cause for your scenario)
      System State backups rely heavily on VSS and Windows Server Backup. Failures during transfer to Azure are often triggered by:
      • VSS writer time‑outs:
        If CPU or memory is heavily used, or another backup product is using VSS at the same time, the VSS writer can time out. Symptoms include MARS errors referencing VSS and Application event log entries such as:
        “A VSS writer has rejected an event with error 0x800423f2, the writer's time‑out expired between the Freeze and Thaw events.”
        Resolution: wait until CPU/memory load is lower or stop high‑usage processes, ensure no other backup is running, then retry the System State backup.
      • Insufficient disk space for shadow copies:
        System State backup needs enough free space on the volume containing system files (typically C:) for VSS shadow copies. If there isn’t enough space, you see volsnap events like:
        “There was insufficient disk space on volume C: to grow the shadow copy storage… all shadow copies of volume C: are at risk of being deleted.”
        Resolution: free disk space on the affected volume and, if needed, adjust shadow copy storage using vssadmin resize shadowstorage as described in the documentation.
      • EFI partition locked:
        If a third‑party security or backup product locks the EFI system partition, the MARS System State backup fails with an error that the EFI partition is locked.
        Resolution: configure the security/backup software to allow MARS access, or wait until the other backup completes and then rerun the backup.
      • Incorrect System State file specification:
        If the file specification used by Windows Server Backup for System State enumeration is corrupted or incorrectly updated, System State backup fails with path‑syntax errors (for example, errors mentioning “The filename, directory name or volume label syntax is incorrect” with HResult 80780049 / 8007007B).
        Resolution steps for that specific condition are detailed in the System State troubleshooting article.
    3. Scratch folder and local storage problems
      Several documented MARS errors (for example, checksum store initialization failures, VHD initialization errors, low disk space, bitmap errors) are caused by issues with the scratch folder:
      • Scratch folder not valid or not accessible to the MARS agent.
      • Scratch folder or its files locked by antivirus or other processes.
      • Not enough free space on the scratch volume.
      These conditions can cause jobs to fail during data transfer even if the error text does not explicitly mention the scratch folder.

    Practical next steps on this server:

    1. Run through the “Before you start” checklist for the MARS agent (agent version, connectivity, services, scratch‑folder free space and accessibility, antivirus exclusions, OS updates, TLS 1.2, .NET).
    2. Check Application and System event logs around the backup time for:
      • VSS writer errors (time‑outs, writer failures).
      • volsnap warnings/errors about insufficient shadow copy space.
      • Windows Server Backup/System State errors mentioning invalid paths or locked partitions.
    3. Ensure no other backup product or security software is running at the same time or locking system volumes/EFI partition.
    4. Free disk space on system volumes (especially C:) and verify that shadow copy storage is sufficient.
    5. After addressing any VSS/volsnap/EFI or scratch‑folder issues found in the logs, rerun the System State backup.

    If the job still fails with the same “unexpected failure while transferring data” after these checks and corrections, the guidance is to collect the MARS logs and relevant event log entries and contact Microsoft support, as the remaining issue is likely an environment‑specific transfer or VSS problem that requires deeper analysis.

    References:

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