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This is a complex issue which has many possible symptoms.

At its core, random files will fail backup. With each subsequent backup run, the number of files that fails to be backed up will change. The same files will not always fail backup.

Further testing with other backup solutions, such as NTBackup, Windows Server Backup, or any of our competitors will also have random files fail backup.

NTBackup and Windows Server Backup will often report the failed fails as corrupted.

The ZCB log files reveal the core error: The parameter is incorrect.


This article does not apply unless random files fail backup in all VSS-based backup solutions.

What's going on?

There is a problem at the system level that results in random failures to back up individual files. As noted above, the number of files that fail will change between backups, and the same files will not necessarily fail each time.

The most common cause is a lack of space for VSS snapshots on the volume that houses the files.


Free space on the volume that houses the files

VSS snapshots require free space. Free space of no less than 10% of the size of the volume is required. Free space of 20% is recommended.

Please create some free space. Common methods involve deleting files and uninstalling programs. Advanced options, such as limiting the size of page files (or moving them to another drive) may also be valid, at your discretion.

Move the VSS snapshots to another drive

If freeing space is not an option, or if enough free space cannot be created, then VSS snapshots can be moved to another drive via the command prompt using the command vssadmin add shadowstorage

Please see this Microsoft article for details:

The drive to which you move the snapshots should meet the following:

  • The drive should be a local drive.
    • External drives may work, but can cause other problems.
    • Network drives cannot house your snapshots.
  • The drive should have ample free space of at least 10% of the size of the original volume. Free space of 20% is recommended.
    • Example: You are backing up C, which is 100GB in size and wish to move the snapshots to D.
      • D needs at least 10GB of free space (10% of the 100GB C drive).
      • Free space of 20GB (20% of the 100GB C drive) is recommended. 

If the problem continues

Creating more free space and/or moving the VSS snapshots may resolve the issue.

 If not, this is a system-level issue that affects all backup solutions. Resolving it is out of the scope of Zmanda Support.

Please contact Microsoft Support.