How to fix Error 521 on Windows 11

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Windows 11 Error 521: Volume Identity Mismatch Troubleshooting Guide

Why This Error Happens

Error 521 typically occurs during Volume Shadow Copy Service (VSS) operations, such as System Restore, backups, or disk snapshot creation, when Windows detects that the volume’s identity (its unique signature or GUID) has changed unexpectedly since the operation began. This is commonly caused by a mismatch between the volume’s tracked state and its actual state, often due to disk changes made outside of VSS awareness (like third-party disk tools, dynamic disk reconfiguration, or abrupt volume resizing), corrupted VSS metadata, or conflicting shadow copy providers. Interrupted previous shadow copy operations or stale VSS writer/provider states can also leave the system referencing an outdated volume identity.

Step-by-Step Resolution Guide

  1. Check VSS Service Status and Restart It
  • Press Win + R, type services.msc, and press Enter.
  • Locate Volume Shadow Copy, right-click it, and select Restart. If it’s not running, set it to Manual and start it.
  1. Verify and Repair VSS Writers
  • Open Command Prompt as Administrator.
  • Run vssadmin list writers and check for any writer showing a Failed or Timed out state.
  • If any writer shows errors, restart the associated service (e.g., restart COM+ Event System or Microsoft Software Shadow Copy Provider) or reboot the system to reset writer states.
  1. Delete Corrupted Shadow Copy Storage
  • In an elevated Command Prompt, run:

    vssadmin delete shadows /all
  • This clears out any stale or corrupted shadow copies that may be causing the identity mismatch. Reboot afterward.
  1. Re-register VSS DLLs
  • In an elevated Command Prompt, run the following commands sequentially:

    net stop vss
    regsvr32 /s vss_ps.dll
    regsvr32 /s /i swprv.dll
    regsvr32 /s /i eventcls.dll
    regsvr32 /s /i es.dll
    regsvr32 /s /i stdprov.dll
    regsvr32 /s /i vssui.dll
    regsvr32 /s /i msxml.dll
    regsvr32 /s /i msxml3.dll
    regsvr32 /s /i msxml4.dll
    net start vss
  1. Check Disk for Errors and Confirm Volume Integrity
  • Run chkdsk C: /f /r (replace C: with the affected volume) via an elevated Command Prompt and schedule it on next restart if prompted.
  • Reboot to allow the check to complete, which will fix underlying file system inconsistencies affecting volume identity.

Alternative or Verification Steps

  • After completing the steps, re-attempt the original operation (backup, restore, or snapshot) and monitor Event Viewer (Applications and Services Logs > Microsoft > Windows > VSS) to confirm no new Error 521 entries appear.
  • If the issue persists, consider creating a new shadow storage association using vssadmin add shadowstorage on a different volume, or perform a System File Checker scan (sfc /scannow) followed by DISM /Online /Cleanup-Image /RestoreHealth to rule out deeper system corruption.