Backup Failing on C drive

If the failing item is C: in Azure Backup, the most relevant cause is insufficient disk space for VSS shadow copies on the system volume, even when the drive appears to have free space overall.

Use this check-and-fix path:

1. Check the VSS-related error details
   • In the backup job details, open the error information for the failed C: item.
   • In Event Viewer, check System logs for volsnap events around the failure time.
   • If the event says there was insufficient disk space on C: to grow shadow copy storage, the failure is VSS shadow-copy growth, not general free-space reporting.
2. Free space on C:
   • Remove unnecessary temporary files and large hidden files.
   • To find the largest files from PowerShell:

          Get-ChildItem -Path "C:\" -Recurse -ErrorAction SilentlyContinue | Sort-Object Length -Descending | Select-Object FullName, Length -First 20
     

   • Or from Command Prompt:

          dir C:\ /s /a /o-s
     

   • Back up needed data before deleting files.
3. Check shadow copy storage usage
   • Run Command Prompt as administrator.
   • Check current shadow storage:

          vssadmin list shadowstorage
     

   • This shows how much space Volume Shadow Copy is using and its maximum allowed size.
4. Adjust shadow copy storage if needed
   • If shadow copy storage is constrained too tightly, resize it.
   • Example:

          VSSAdmin Resize ShadowStorage /For=C: /On=C: /MaxSize=1GB
     

   • This changes the maximum space available to shadow copies. Use a value appropriate for the server.
   • Warning: changing shadow copy storage can affect restore points or shadow-copy retention.
5. Retry when no other backup is running
   • If another backup product is using VSS, Azure Backup can fail while creating the snapshot.
   • Make sure no other backup job is active, then run the backup again.
6. If large temporary files keep returning
   • Identify the application creating them.
   • Recently used applications and continuously running applications are the main suspects.

If support escalation is needed, collect:

• Windows Server version and build (winver)
• Installed applications and versions
• Largest file paths and timestamps on C:
• Directory size report
• Event Viewer logs covering the time disk space started dropping or the backup failed

• Drive C warns about low disk space but Windows Server shows available space
• Why the used space of C drive increases rapidly while using Vista?
• Troubleshoot System State Backup