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This article applies if:

  • You are running a Backup to Cloud operation.
    • This article does not apply to a Backup to Disk and Cloud or an Upload from Disk.
  • Your Backup to Cloud fails after running for a while.
  • One or more of the following is true:
    • Your backup is large.
    • Your upload bandwidth is slow.
    • The backup takes a long time.
    • Your machine is under heavy load.
  • You may encounter errors such as "VSS snapshot deleted" or "VSS snapshot failed".

What's going on?

ZCB uses Volume Shadow Copy (VSS) snapshots to perform backups. Your machine must keep the snapshot intact during the entire duration of your backup.

For backups to disk, this is usually a short period of time. A few minutes, usually, or sometimes a few hours for exceptionally large backups. After the backup to disk is complete, the snapshot is released. Then ZCB uploads the completed backup to the cloud.

For a backup direct to the cloud, the snapshot must be active and intact for the entire duration of the backup to the cloud. This can be a very long time, especially for large backups on a slow connection.

While a snapshot is active, it must account for all data written to the disk. The more data is written, the larger the snapshot grows. Snapshots are allocated a limited amount of space by the system. If a snapshot grows larger than this allocated space, it "breaks". The backup fails.

Backups to Cloud keep the snapshot open for a long time. Long enough that even a light load on the machine can cause the snapshot to break. A machine under a heavy load will cause the snapshot to break faster.

Database backups, such as MS-SQL and Exchange, are far more prone to this sort of failure than File System backups.


As a best practice, always perform a Backup to Disk and Cloud for any backup set that is large and/or takes a long time to upload.

Also, consider dividing your data into multiple backup sets. This has two advantages: it makes each backup image smaller, and it allows you to have different backup policies on different kinds of data.


If your Backup to Cloud fails after a while, try a Backup to Disk and Cloud. If the Backup to Disk and Cloud is successful, you have likely run afoul of the problem described above.

If the Backup to Disk and Cloud also fails, please open a support case.