Fix 0XC01C0011: Filter Driver Altitude Collision

When This Error Shows Up

You're setting up an encrypted volume or mounting a VHD, and bam — STATUS_FLT_INSTANCE_ALTITUDE_COLLISION with code 0XC01C0011. This usually pops up in Windows 10 20H2 or later, when a third-party filter driver (like an old backup tool or a security product) has already claimed the altitude on a specific volume. I've seen it most with Symantec Endpoint Protection and certain Dell backup utilities.

The trigger: you try to attach a new filter driver to a volume, but the Filter Manager says, "Nope — another instance is already sitting at that altitude." It's like two people showing up for the same seat on a plane.

Root Cause (In Plain English)

Every filter driver in Windows has an altitude — a number that tells the system where it sits in the stack. Think of it like height levels in a parking garage. If two drivers try to park at the exact same level, the system throws this error. The collision happens because:

An old filter driver didn't unload properly after uninstall.
A security product or backup tool grabbed an altitude that conflicts with your new driver.
You're trying to load a custom filter driver with an altitude already in use by a Microsoft or third-party driver.

The Filter Manager registers altitudes in the registry at HKLM\SYSTEM\CurrentControlSet\Services\YourFilter\Instances. When two entries share the same altitude value, you get 0XC01C0011.

The Fix — Step by Step

Skip the reboot. Skip the driver reinstall. The real fix is to find the conflicting instance and remove it. Here's the exact process I've used on hundreds of machines.

Open Command Prompt as Administrator. Type cmd in Start, right-click, and choose "Run as administrator."
List all filter instances on the volume. Run this command, replacing D: with your volume letter:
```
fltmc instances -v D:
```
Look for lines showing Altitude and InstanceName. You're hunting for two entries with the same altitude number.

Identify the conflicting driver. The output looks like:

Filter                Volume Name                              Altitude  Instance Name      Frame
-----                 ----------                              --------  --------------     -----
MyFilter              D:                                      370000    MyFilter-Instance  0
OldBackupFilter       D:                                      370000    OldBackup-Instance  0

Both at altitude 370000 — that's your collision.

Detach the unwanted instance. Run:
```
fltmc detach OldBackupFilter D:
```
Replace OldBackupFilter with the actual filter name from step 3. This removes the instance from the volume without uninstalling the driver completely.
Verify the fix. Re-run the fltmc instances command. You should only see one instance at that altitude now. Then try your original operation — mount the volume, load your filter, whatever triggered the error.

Pro tip from my help desk days: If you don't know which driver to detach, check the Filter column against known software. Third-party backup tools (Acronis, Macrium, Veeam) and security suites (McAfee, Trend Micro) are the usual suspects. Detach the one you didn't install today.

Still Getting the Error? Try This

If fltmc detach fails, the driver instance is persistent — meaning it reloads on boot. You'll need to kill it at the service level:

Open Services.msc (Win+R, type services.msc).
Find the service tied to the filter driver. Look for something matching the filter name from fltmc output. For example, Symantec Endpoint Protection or Macrium Reflect Image Mount Driver.
Stop the service, then set its startup type to Disabled temporarily.
Reboot and try your operation again. If it works, you know the service was the culprit. You can re-enable it after your operation, or contact the vendor for a fix.

On rare occasions, the conflict lives in the registry. Pop open Regedit and go to HKLM\SYSTEM\CurrentControlSet\Services\YourFilter\Instances. If you see duplicate altitude keys under different instances, delete the one you don't need. Backup the key first — I've had to restore it for someone who deleted the wrong one.

One last check: Run fltmc filters to list all loaded filters. If you see something like BindFlt (part of Windows) at the same altitude as yours, you've got a system collision. Don't touch BindFlt — instead, change your filter's altitude in its INF file or registry to an unused range. Microsoft publishes official altitude ranges. Use a free one in the 360000–380000 range for third-party drivers.

If none of this works, you're probably looking at a corrupted filter driver installation. Uninstall the conflicting software completely, clean the registry of leftover filter entries using Autoruns from Sysinternals, then reinstall. This error is stubborn, but not unbeatable.