Symantec Endpoint Protection client is preventing Windows system backup
Various errors and backup products. For example, Windows System State Backup fails when SEP is installed. If SEP is removed the issue does not occur. The error message presented is "failed to format media".
Symantec Endpoint Protection interferes with unmounting of storage after a backup is done. Issues surrounding this were fully fixed in SEP 14.0.
Upgrade to Symantec Endpoint Protection version 12.1 RU6. As of SEP 12.1 RU6, SymEFA was changed to track volume lock requests to prevent interference when Auto-Protect closes a volume handle. There are still some reports of similar symptoms in RU6; for a full fix to this issues upgrade to SEP 14.0.
A work-around may also be implemented by disabling the SymEFA. Disabling this driver and all its attendant services essentially disables Advanced Download Protection and Proactive Threat Protection. Please contact Symantec technical support if you need further assistance with this issue.