Fix STATUS_FLT_VOLUME_ALREADY_MOUNTED (0xC01C001A)

When This Error Hits

You're trying to mount a volume in Windows — maybe through Disk Management, maybe via a script — and you get the error STATUS_FLT_VOLUME_ALREADY_MOUNTED (0xC01C001A). This usually happens when you've got a volume that's been mounted before and something left a stale mount point. I've seen this after a failed backup restore on Windows 10 22H2, or when a USB drive gets unplugged without safely removing it and then plugged back in. The system still thinks the volume is mounted, but you can't see it in File Explorer.

What's Causing It

The volume is already mounted somewhere in the system's memory — but the mount point (drive letter or folder) is gone or broken. It's like the door is locked from the inside, but nobody's home. The file system filter manager (FltMgr.sys) is holding onto a reference that says "this volume is in use." Trying to mount it again gives you that exact error code.

The Fix: Step by Step

You'll need to open Command Prompt as Administrator for this. Press the Windows key, type cmd, right-click Command Prompt, and pick Run as administrator.

Step 1: Find the Volume Number

1. In the Command Prompt, type diskpart and press Enter. DiskPart starts up.
2. Type list volume and press Enter. You'll see a table with all volumes.
3. Look for the volume that's giving you trouble. It might not have a drive letter. Write down the volume number (like Volume 3).

Expected outcome: You see a list of volumes. The one you want might show "No" under Ltr (letter). That's normal.

Step 2: Select and Remove the Mount Point

1. Type select volume X (replace X with your volume number) and press Enter. DiskPart confirms: "Volume X is the selected volume."
2. Type remove all dismount and press Enter. This forces the volume to dismount and removes any mount points.

Expected outcome: DiskPart says "DiskPart successfully removed the drive letter or mount point." If it says "There is no mount point," that's fine too — the volume is already stripped of letters.

Step 3: Assign a Fresh Drive Letter

1. Type assign letter=Z (pick a letter you know is free) and press Enter.
2. DiskPart says "DiskPart successfully assigned the drive letter or mount point."

Expected outcome: The volume now shows up with a drive letter. You can check in File Explorer or with list volume again.

Step 4: Exit DiskPart and Test

1. Type exit to leave DiskPart.
2. Type exit again to close Command Prompt.
3. Open File Explorer. You should see the volume under This PC with the letter Z (or whatever you used).

Expected outcome: The volume mounts correctly. No more error 0xC01C001A when you try to access it.

If That Didn't Work

Sometimes the issue is a stuck service. Restart the Filter Manager service:

1. Open Command Prompt as Administrator again.
2. Type net stop fltmgr and press Enter. It might say the service is not running — that's okay.
3. Type net start fltmgr and press Enter.
4. Try the steps above again from Step 1.

If it still fails, check the volume for corruption. In Command Prompt, run chkdsk Z: /f (replace Z with your drive letter). Let it finish and try again.

One last thing: if this keeps happening on the same USB drive, the drive might be dying. I've seen bad flash drives cause this error every time you plug them in. Replace the drive if chkdsk finds a lot of bad sectors.