• Volume Shadow Copy Service (VSS) is a Microsoft technology for taking consistent, point-in-time snapshots of a disk volume. A VSS Writer is an extra software plugin that detects when this action is taking place, and ensures that application-specific files are in a safe state on disk. This feature allows you to invoke a single VSS Writer, or a sub-component of a single VSS Writer, and to backup only the files that it was protecting. 
• VSS snapshotting does not work over network paths. Instead of trying to snapshot over a network, install the client software onto the device which holds the original data.
• VSS issues can occur if another backup software is also running, and has prevented the GBM VSS writer from working. Stop the other software to see if the issue persists.
• VSS issues are often accompanied by an error code in the form of 0xXXXXXXXX. This is an externally-originating code, which will often signify why the VSS writer failed. Research this error code for solutions to the problem.
• Open an elevated-permissions command line, and run “vssadmin list writers” to see a current list of the writers. This will highlight any which failed, as well as their own error codes. This may help troubleshoot which writer contributed to the failure, or show the failure point.