Error Message:

VSS open files Backup failed. Error: The maximum number of volumes has already been added to the shadow copy set. in BCVssOpenFilesProcess

The Error is thrown by VSS (Volume Shadow Copy Service). This error is caused due to the following reasons:

- Not enough disk space available to create a shadow copy of the files. Make sure you have enough free space (Temp Path location for the Endpoint).

If you do not have enough free space, redirect the Temp Path location on the Endpoint to a bigger drive, or free some space by deleting Temporary files and running Disk Cleanup. You can also create more free disk space by deleting all restore points except the latest:

http://support.microsoft.com/kb/555367

- Some other software may also be trying to access the shadow volume for that drive, e.g. disk defragmenters, other backup software. If you have other software installed that maybe be using VSS, reschedule the backup to a different time.