ANSWER  VSS stands for Volume Snapshot Service or Volume Shadow Copy Service. Let us explain...

VSS is a technology developed by Microsoft that allows backup applications to safely back up locked and open files. Both the backup application and software application must be VSS-aware in order to utilize VSS to back up open or locked files.

Some VSS limitations:

  • Only local disks are supported
  • The software program that created the open or locked files must support VSS.
  • Microsoft Volume Shadow Service must be enabled in the OS (it is enabled by default)
  • The system must have an NTFS partition that exists on an internal drive, with at least 100MB of free space for the shadow copies
  • The drive that contains locked files must have free space available.

IMPORTANT  VSS can fail if there is too much disk activity.

For more information, please refer to this Wikipedia article.