1. Lack of disk space available for taking the snapshot. 10-15% free disk space is crucial for good VSS performance.
2. A disk with less than 10% free space left is likely to run 50% slower when data is added or updated frequently.
3. Apart from speed issues, VSS requires a lot of temporary space, too. When backups run for a long time, disk write accesses (changed disk blocks) need to be buffered in a dedicated area on disk. That area is known as “VSS Storage Area” and keeps growing until the backup finishes; hence, the longer the backup takes and the more data is being updated during backups, the more space is needed for VSS to work properly.
1) Check free disk space on virtual machines.