Fix 0x801F0012 ERROR_FLT_INSTANCE_NAME_COLLISION

You're seeing this when a filter driver tries to register a duplicate instance name on a volume. Quick fix: remove stale registry entries or disable the conflicting filter.

Quick Answer

Run fltmc instances to find the duplicate instance name, then fltmc detach instance-name volume to unload it. If that fails, delete the registry key under HKLM\SYSTEM\CurrentControlSet\Services\<FilterDriver>\Instances.

Why This Happens

This error means a filter driver — like an antivirus scanner, backup software, or encryption tool — tried to create an instance (a filter attachment) on a volume, but another instance with the same name is already registered there. I see this most often after a botched software uninstall or a driver update that left stale entries in the registry. Had a client last month whose backup software (Acronis) left a ghost instance on their C: drive after an update failed mid-way. Every time Windows booted, the driver service tried to register its instance, hit the collision, and tossed up this error. The volume usually still works, but the filter won't load — meaning no real-time scanning or encryption on that drive.

Fix Steps

Open Command Prompt as Admin — hit Win+X, select Terminal (Admin) or CMD (Admin). Don't skip this, you need elevation.
List all filter instances on the problematic volume — run fltmc instances and note the Volume column. Look for your volume (e.g., C:) and see if any instance name appears twice.
Identify the collision — if two entries show the same InstanceName on the same volume, you've found the culprit. The error code confirms it.
Detach the duplicate instance — run fltmc detach <InstanceName> <Volume>. For example, fltmc detach AcronisEncFilter C:. This unloads the filter driver from that volume without a reboot.
Verify the fix — run fltmc instances again. Only one instance of that name should remain on the volume.
If detach fails — some drivers won't let you detach. In that case, open Regedit, go to HKLM\SYSTEM\CurrentControlSet\Services\<DriverName>\Instances, find the duplicate instance entry, and delete it. Backup the key first.

Alternative Fixes

If the above doesn't clear it, try these in order:

Unload the entire filter driver — fltmc unload <DriverName>. This removes all instances of that driver. You'll need to restart the driver's service after.
Disable the filter driver — in Services.msc, find the driver's associated service (e.g., Acronis Active Protection), set it to Disabled, reboot, then re-enable it. This forces a clean state.
Use Diskpart to reset volume — if the volume itself is corrupted, diskpart -> select volume X -> detail volume -> check for issues. But this is rare — 99% of the time it's a stale instance.

Prevention Tip

Before uninstalling any filter driver software (antivirus, backup, encryption), always stop the service first, then uninstall from Control Panel. Don't force-remove the driver files — that leaves orphaned instance entries. Also, after updates, run fltmc instances to spot duplicates early. Takes 10 seconds and saves you this headache.