Quick answer: Run fltmc instances to find the conflicting instance, then delete it with fltmc detach or nuke the stale registry key in HKLM\System\CurrentControlSet\Services\FltMgr\Instances.
What's actually happening
This error means two filter drivers (like antivirus, backup software, or encryption tools) registered with the same instance name on the same volume. Windows' Filter Manager (FltMgr) won't allow duplicate instance names – it's a hard rule. I've seen this most often after a botched uninstall of Symantec Endpoint Protection or Acronis True Image. The uninstaller leaves a registry key behind, but the volume still thinks the filter is loaded. Next time you try to mount the volume or load a new filter with that name, bang – 0xC01C0012.
A client last year had this on a server with an old McAfee driver hanging around after an upgrade to Windows Server 2019. Every time they tried to attach a new antivirus, the volume rejected it. Took me 20 minutes to dig out the orphaned instance.
How to fix it
Step 1: Identify the conflicting instance
Open an elevated command prompt (run as Administrator). Type:
fltmc instancesLook for any instance that shows the same name on the same volume. You'll see the volume name (like C:) and the instance name (like "Symantec"). If you see duplicates, that's your problem.
Step 2: Detach the bad instance
If you find a duplicate, detach it. The syntax is:
fltmc detach <instance_name> <volume>For example:
fltmc detach Symantec C:This removes the filter instance from that volume. The volume should then accept a new instance with the same name. If it fails with "access denied", the filter is still loaded – you'll need to unload the driver (see step 3).
Step 3: Unload the driver if detach fails
Sometimes the driver is still active. Check with:
fltmc filtersFind the filter name. Then unload it:
fltmc unload <filter_name>If the unload fails (common with antivirus drivers that protect themselves), you'll need to reboot into Safe Mode and try again. Or skip to the registry fix below.
Alternative fix: Nuke the registry entry
When fltmc commands won't work, go straight to the registry. Open Regedit and navigate to:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\FltMgr\InstancesEach subkey here is a filter instance. Look for the one with the conflicting name. Right-click and delete it. Then reboot. Windows will rebuild this key from the filter drivers that are actually installed. This is the nuclear option – I've used it on dozens of machines where the uninstaller was garbage.
Warning: Don't delete the entire FltMgr key – only the offending instance subkey inside Instances. Back up the key first if you're nervous.
Prevention tip
Before uninstalling any security or backup software, use the vendor's official removal tool (like Symantec CleanWipe or Acronis Cleanup Utility). Those tools are designed to wipe the filter driver and its instances. If you skip that step, you're leaving landmines. Also, after uninstalling, always run fltmc instances to verify no orphaned instances remain. Takes 10 seconds and saves you an hour later.
If you're adding a new filter driver to a volume that's had previous software, manually remove any old instances first. The new installer won't always clean them for you.