Fix ERROR_FLT_DISALLOW_FAST_IO (0x801F0004) on Windows

Cause #1: A rogue filter driver (most common)

This error pops up when a file system filter driver—usually from antivirus, backup software, or a security tool—decides to block the Fast IO path. Fast IO is Windows’ high-speed lane for file operations (think copying large files or reading databases). When a filter driver says “nope,” you get 0x801F0004.

I saw this last week with a client running a legacy Symantec Endpoint Protection on a Windows Server 2019. The AV driver (symefasi.sys) was flagging every file write as suspicious. Disabling it fixed the issue in under 5 minutes.

Fix: Identify and disable the culprit filter

1. Open an elevated Command Prompt (Run as Administrator).
2. Run fltmc filters – this lists every filter driver stacked on the system. Look for anything that isn’t Microsoft – those are your suspects.
3. Check version info: fltmc instances shows which filters are loaded per volume.
4. For each third-party filter, try: fltmc unload <FilterName> (e.g., fltmc unload symefasi). The filter will detach immediately. If the error disappears, you found it.
5. Permanently disable it via sc config <FilterName> start= disabled or remove the associated software.

Real talk: Some filters (like Malwarebytes or Carbon Black) protect themselves from unloading. In that case, uninstall the product, reboot, and reinstall without the real-time file scanning component if you can.

Cause #2: Corrupt or misconfigured antivirus

Antivirus tools hook into the file system via minifilter drivers. A corrupted driver file or a botched update can cause the filter to block Fast IO on every operation. Had a client last month whose entire print queue died because of this – the AV filter was blocking spool file writes.

Fix: Reset or reinstall the antivirus

1. Disable real-time protection temporarily. If the error stops, that’s your sign.
2. Use the vendor’s removal tool (e.g., McAfee MCPR, Symantec CleanWipe) to wipe the driver completely.
3. Reboot. The filter should be gone. Run fltmc filters again to confirm.
4. Reinstall the latest version of the antivirus. Stick with the default settings – don't enable “aggressive” scanning unless you need it.

Pro tip: If you’re stuck with a corporate-managed AV (like CrowdStrike), you might not be able to remove it. Contact your admin and ask them to exclude the volume or folder where the error happens. I’ve seen this work for SQL Server tempdb writes.

Cause #3: A bad third-party backup or sync tool

Tools like Veeam, Acronis, or even Google Drive for Desktop install their own filter drivers to intercept file changes. If they crash or have a version mismatch, they’ll block Fast IO instead of passing it through.

Fix: Unload the backup filter and reconfigure

1. Check which backup software is installed. Look in Control Panel > Programs and Features.
2. Run fltmc filters and look for names like “VeeamFlt” or “AcrFlt”.
3. Unload it with fltmc unload VeeamFlt (adjust the name). If the error clears, you've got your culprit.
4. Update the backup software to the latest version. Many old builds have bugs that block Fast IO.
5. Reinstall the backup tool and reconfigure it – make sure it’s not filtering the volume where the error occurs (e.g., exclude C:\Windows or your data drive).

Story from the field: A client using CrashPlan for business had this error every time they copied files to a NAS. CrashPlan’s filter driver (crpflt.sys) was causing the conflict. Updating CrashPlan from version 6.8 to 6.9 fixed it instantly. Always check for updates first.

Quick-reference summary table

If none of these work, you might be dealing with a hardware issue (bad disk controller) or a corrupt system file. Run sfc /scannow and chkdsk /f on the drive. But 9 times out of 10, it’s a filter driver. Start there and you’ll save yourself an hour of head-scratching.