Problem description
SQL backup jobs fail intermittently, reporting a generic 'Internal 65535' error within the management console. This issue typically occurs during the data upload phase of the backup process. While the connection to the SQL instance is successful, the actual transfer of database files fails because the underlying volume snapshot cannot be maintained.
Cause
The root cause is insufficient shadow copy (VSS) storage space on the drives hosting the SQL database files.
During a backup, the Volume Shadow Copy Service (VSS) creates a snapshot to ensure data consistency. If the disk experiences high I/O or if the database is large, the "diff area" (storage for shadow copies) may run out of space or fail to grow fast enough, causing the snapshot to be deleted and the backup job to fail.
Traceback
Detailed job logs (Enterprise Workloads) will show the following upload failures:
Plaintext
[2025-10-02 00:06:54,094] SQL GO Uploader failed to upload DB file test1.mdf, File upload status: FILE_UPLOAD_FAILED [2025-10-02 00:06:54,094] SQL GO Uploader failed to upload DB file test2.mdf, File upload status: FILE_UPLOAD_FAILED
Troubleshooting Steps:
Search the log file for the database names to identify the file paths:
[[u'D:\\data', u'test1.mdf', 0] — This confirms the database resides on the D: drive.
Check the Windows System Event Logs for Volsnap errors:
Log Name | Source | Event ID | Description |
System | Volsnap | 25 | The shadow copies of volume D: were deleted because the shadow copy storage could not grow in time. |
Resolution
To resolve this, you must increase the shadow copy storage limit for the affected volume. It is generally recommended to allocate 10%–15% of the total volume size for shadow copies.
Steps to increase Shadow Copy space:
Check Disk Space: If the drive is critically low on free space, extend the physical drive size first.
Open Properties: Open File Explorer, right-click the affected drive (e.g., D:), and select Properties.
Configure Shadow Copies: Navigate to the Shadow Copies tab (or right-click the drive and select Configure Shadow Copies).
Modify Settings:
Select the volume from the list.
Click Settings.
Under Maximum size, select Use limit and enter a higher value, or select No limit (UNBOUNDED) if disk space allows.
Save: Click OK to apply changes.
Retry: Re-run the SQL backup job.
Verification
To verify the resolution:
Run the command
vssadmin list shadowstoragein an elevated Command Prompt to confirm the new limits are applied.Monitor the System Event Log during the next backup cycle to ensure no Volsnap Event ID 25 or 35 errors appear.
Confirm the backup job status in the management console shows Successful.