Windows Will Block ReFS USB Drives From January 2027: How to Check If You're Affected

Microsoft is about to block a type of external drive in Windows. From the January 2027 security update, Windows 10 and 11 will refuse to open USB sticks and external drives formatted with ReFS, Microsoft's newer file system, unless an administrator turns the block off. Most people won't notice, because most drives use NTFS, exFAT or FAT32. But if one of yours uses ReFS, you need to act before January or lose access to it.

What is changing?

Microsoft says ReFS on removable media, "such as a USB disk", is "not a supported configuration", and it's rolling out the change in two steps, set out in its support note KB5130741 of 13 October:

Now (since the October 2026 update): Windows only warns. When it finds a ReFS volume on a removable or externally attached drive, it logs a warning, and Windows Latest found an alert under the removable device icon in the taskbar too. The drive still works.
From the January 2027 security update: blocking is on by default. Windows won't mount ReFS volumes on removable or external drives, so they won't show up and you can't open your files.

The block doesn't delete anything, as Windows Latest points out: the data stays on the drive, Windows just won't open it. Microsoft frames the change as a security hardening step.

Am I affected?

Only if a USB stick or external drive is formatted as ReFS. Drives formatted as NTFS, exFAT or FAT32, which is almost all of them, aren't affected. Internal drives aren't affected either, including Windows 11's Dev Drive, which also uses ReFS.

To check a drive:

Plug it in and open File Explorer.
Right-click the drive and choose Properties.
Look at "File system" on the General tab. If it says ReFS, you're affected.

On a PC with several drives, you can also check Event Viewer: open Windows Logs, then System, and filter for Event ID 524. Since the October update, Windows logs this warning, from the source "ReFS", once per affected volume each time the PC starts, with a message saying the volume "is on a removable or externally attached disk".

Which versions of Windows?

Every supported version, according to Microsoft: Windows 10 22H2 and the Windows 10 Enterprise LTSB/LTSC editions, Windows 11 from 22H2 up to 24H2, 25H2 and 26H1, and Windows Server 2012 (with extended security updates) through to Windows Server 2025.

What should I do before January?

Microsoft's advice is to move the data to "a supported storage configuration". In practice:

Copy everything from the ReFS drive to another drive or your PC.
Reformat the external drive as NTFS (or exFAT if you also use it on a Mac or a TV).
Copy the files back.

Do it before the January update arrives, while Windows can still open the drive. If you're moving to a new PC anyway, Windows 11 no longer has its own PC-to-PC transfer tool, so here's how to move your files without OneDrive.

Can I turn the block off?

Yes, but it's meant for administrators, and Microsoft still calls the setup unsupported. The control is a setting called RefsRemovableDeviceMountPolicy, available as a Group Policy and as a registry value:

0: off (no warning, no block)
1: audit, the current default (warn only)
2: block, the default from January

The registry value lives under HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\FileSystem; the Group Policy version is stored under HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Policies and wins if both are set. You need to restart the PC after changing either. Editing the registry can break Windows if you get it wrong, so moving the data is the safer fix for most people.

Why is Microsoft doing this when it's pushing ReFS?

It looks odd, as Windows Latest notes. ReFS (Resilient File System) is Microsoft's successor to NTFS, built for data integrity and huge volumes up to 35 petabytes. Windows 11 already uses it for Dev Drive, and Microsoft is testing an option to install Windows itself on ReFS in preview builds. But Microsoft has never supported ReFS on removable drives, and it's now enforcing that line for security, while ReFS keeps growing on internal drives.