"SQL9: SQL Agent internal error occurred" during MS SQL Server Backups

Problem description

When attempting to back up a Microsoft SQL Server instance using the Druva Enterprise Workloads agent, the backup job fails during the file-estimation phase.

The following errors are observed within the Druva progress logs or the Druva Management Console:

Druva Error: SQL9: SQL Agent internal error occurred
Operating System Error: CreateFile ... The network path was not found.

Associated Symptoms:

Differential backups fail continuously, which subsequently causes upcoming Transaction Log (T-log) backups to be skipped.
Manual backup attempts fail immediately with the identical error signature.

Cause

This issue is caused by an environmental conflict stemming from residual configurations, stale registry keys, or leftover virtual disk drivers belonging to a legacy, third-party backup agent previously deployed on the host.
During the file-estimation phase, the Druva backup pipeline attempts to query and map database files.
If residual components exist, the system may trace file structures through a stale virtual volume path mapped to the legacy software's device interface (for example: \\GLOBALROOT\Device\[LegacyBackupVirtualDisk]\...).
Because the legacy software has been partially uninstalled or is inactive, this network path breaks mid-operation. The Operating System returns a "network path not found" error, resulting in the agent dropping the operation and reporting a SQL9 execution failure.

Traceback

[YYYY-MM-DD HH:MM:SS,x] [ERROR] [thread_id]  Error in SQL backup operation. Code: SQL9. Message: SQL Agent internal error occurred. [YYYY-MM-DD HH:MM:SS,x] [DEBUG] [thread_id]  OS Error during CreateFile mapping for path: \\GLOBALROOT\Device\[LegacyBackupVirtualDisk]\MSSQL\DATA\db_file.mdf -> Error Code 3: The network path was not found. [YYYY-MM-DD HH:MM:SS,x] [ERROR] [thread_id]  File estimation phase failed. Skipping subsequent Transaction Log backups due to broken chain verification.

Resolution

To resolve this error and clear the stale paths, perform the following troubleshooting steps:

Step 1: Purge Legacy Agent Remnants

Inspect the host server's Apps & Features or Program and Features control panel to confirm full uninstallation of prior backup tools.
Open the Device Manager, enable Show hidden devices under the View menu, and inspect Storage volumes or System devices for any lingering virtual disk drivers tied to the legacy software. Uninstall them if found.
Check the host's Windows Services (services.msc) and ensure no legacy backup services remain.

Step 2: Trigger a Manual Validation Backup

Select the backup set.
Click Backup Now to trigger an immediate manual backup.
Monitor the progress logs to ensure the file-estimation phase completes successfully without encountering virtual disk mapping errors.

Verification

To verify the resolution:

Confirm that the newly initiated manual backup completes with a Successful status in the Druva Console.
Open the backup progress log and verify that the file-estimation phase correctly maps local drives (e.g., C:\Program Files\Microsoft SQL Server\...) rather than resolving to a \\GLOBALROOT\Device\... legacy pathway.
Verify that the subsequent scheduled Differential and Transaction Log backups run automatically without being skipped.