How to fix Error 421 on Windows 11

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Troubleshooting Guide: Error 421 – “This Stream Is Not DAX Mappable”

Why This Error Happens

Error 421 occurs when the Windows Direct Access (DAX) mapping mechanism attempts to memory-map a file stream that does not meet the required alignment or format criteria, typically because the underlying storage volume is not formatted with Persistent Memory (PMem) support or the file was not created with the FILE_ATTRIBUTE_ALIGNED_MAPPED_FILE flag. This commonly happens when applications try to DAX-map files residing on non-DAX volumes, compressed/encrypted files, or streams from an NTFS alternate data stream. Corrupted volume metadata or a mismatch between the storage driver and DAX-aware file system configuration can also trigger this error.

Step-by-Step Resolution Guide

  1. Verify DAX Volume Support
    Open PowerShell as Administrator and run:
   Get-Volume | Select-Object DriveLetter, FileSystem, DaxCapable

Confirm the target drive shows DaxCapable: True. If False, the volume must be reformatted with DAX support enabled.

  1. Reformat the Volume with DAX Support (if unsupported)
    Back up your data, then reformat using:
   Format-Volume -DriveLetter X -FileSystem NTFS -DaxMode Enabled

Replace X with the correct drive letter. Note: DAX mode requires a Persistent Memory (PMem)-capable disk.

  1. Check File Attributes and Compression/Encryption Status
    Navigate to the file location in File Explorer, right-click the affected file → Properties → Advanced, and ensure “Compress contents” and “Encrypt contents” are both unchecked. DAX-mapped streams cannot be compressed or encrypted.

  2. Update Storage and PMem Drivers
    Go to Settings > Windows Update > Advanced options > Optional updates > Driver updates, and install any pending updates for NVDIMM, Persistent Memory, or storage controller drivers.

  3. Run System File Checker to Rule Out Corruption
    In an elevated Command Prompt, run:

   sfc /scannow

Followed by:

   DISM /Online /Cleanup-Image /RestoreHealth

Alternative or Verification Steps

  • Verify the fix: Re-run the application or process that triggered Error 421; check Event Viewer (Windows Logs > Application) to confirm no new Event ID 421 entries appear.
  • Alternative: If DAX mapping isn’t strictly required, configure the application to use standard memory-mapped I/O (non-DAX) instead, bypassing the need for PMem-formatted volumes entirely.