Windows will stop mounting ReFS-formatted USB drives in January

A file system is the filing cabinet on a drive. It decides how files are stored, named and found again. Most Windows PCs use NTFS, which has been the default for decades.

ReFS, short for Resilient File System, is Microsoft's newer cabinet. It was built to protect data from damage and to handle huge volumes. Windows Latest notes it supports partitions of up to 35 petabytes, which is a lot of vacation photos.

Microsoft has been giving ReFS a bigger role in Windows lately:

• Dev Drive: Windows 11's storage space for developer work already runs on ReFS.
• Block cloning: in 2024, Microsoft added native block cloning, Windows Latest reports. Copies on the same ReFS volume can share existing data blocks, and only the changed parts get written again.
• Booting: OSNews points out that, since earlier this year, Windows 11 and Windows Server can boot from ReFS volumes.
• Setup: Windows Latest reports that a ReFS option in Windows Setup is in testing.

Microsoft's support document is short and direct: "ReFS on removable media, such as a USB disk, is not a supported configuration." Windows Latest notes Microsoft has listed removable media as unsupported for a while, even though those drives kept mounting. What's new is that Windows will now enforce it.

The change arrives in two phases.

• October 13, 2026 security update (audit phase): auditing is on by default. Windows notices ReFS volumes on qualifying external media and logs a warning. Blocking is off.
• January 2027 security update (enforcement phase): auditing and blocking are both on by default. ReFS volumes on removable or externally attached disks are blocked from mounting unless an administrator changes the setting.

If nobody touches the setting, ReFS-formatted USB and external drives stop mounting with the January 2027 security update.

After January, the only way to keep such a drive mounting is the new policy setting described below.

Windows already tells you. When auditing is on, ReFS logs a warning, Event ID 524, when it finds a volume on a removable or externally attached disk.

The message in the log says the volume "is on a removable or externally attached disk," that this "is not a supported configuration for ReFS," and that it will not be mounted if a blocking policy is enabled. It ends with a plain instruction: move the volume to a supported disk type.

By the numbers:

• Once per boot: Windows logs the event once for each affected volume during each system boot.
• 32 volumes: Windows tracks up to 32 affected external volumes per boot for auditing.
• No limit on blocking: Microsoft stresses that the 32-volume cap applies only to the audit list. Blocking works no matter how many volumes are recorded.

Windows Latest tested this on a PC with the October update. It saw the warning in Event Viewer and an alert in the system tray under the removable device icon. The drive itself still worked.

In real life Plug in your external backup drive, open Event Viewer and search the System log for Event ID 524 with the source ReFS. No event, no problem. If it shows up, copy what you need to an NTFS or exFAT drive before January.

They stay where they are. Windows Latest notes that the block works at the mounting level. Windows simply won't make the volume available, and it doesn't delete anything on it.

The catch: a drive that won't mount is a drive you can't open. Microsoft's own warning is clear: "Blocking can affect access to data on an external ReFS volume. Ensure that required data is available from supported storage before enforcing the policy."

Think of it like a building that changes its locks. Your stuff is still inside the apartment, but your old key no longer opens the door. Moving the boxes out before the locksmith arrives is the easy option.

Microsoft added one new setting, RefsRemovableDeviceMountPolicy. It can be set through Group Policy or directly in the Windows registry.

A few details matter here:

• Registry location: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\FileSystem.
• Policy location: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Policies.
• Who wins: when the value is set in both places, Group Policy takes precedence. A local registry value does not override it.
• Restart: the device has to restart before a new value takes effect.

What's next: Microsoft lists five steps for organizations. Install the October update, review the audit events, find workflows that rely on ReFS on external media, test blocking in a controlled setup, then enable blocking when ready. If no override exists, January turns blocking on anyway.

Most people will never notice. USB sticks and external drives usually come formatted as exFAT, FAT32 or NTFS, and none of those are part of this change.

The people who will notice are the ones who formatted a removable or external disk as ReFS on purpose. That could be someone who wanted ReFS's data protection on a backup drive, or a team that moves ReFS volumes between machines on external disks.

The change also reaches far. Microsoft's list of affected systems covers:

• Windows 10: version 22H2 and the LTSB 2016, LTSC 2019 and IoT Enterprise LTSC 2021 editions.
• Windows 11: versions 22H2, 23H2, 24H2, 25H2 and 26H1, including SE and IoT editions.
• Windows Server: 2012 and 2012 R2 with Extended Security Updates, plus 2016, 2019, 2022 and 2025.

Microsoft frames it as security hardening. The support document says the change improves the security posture of Windows by adding visibility into unsupported external ReFS setups and by giving admins controls to block them.

It lists four goals: better visibility through audit events, less exposure by letting admins block early, a phased rollout so teams can find what depends on these drives, and central management through Group Policy.

It doesn't go further than that. OSNews observes that the company does not explain what makes ReFS on removable media risky beyond calling it unsupported.

The big picture: this doesn't change Microsoft's plans for ReFS on internal drives. Dev Drive keeps using it, booting from ReFS keeps working, and the Setup option is still in testing. Windows Latest notes it is still unclear when, or whether, ReFS will replace NTFS as the default. One possibility it raises is that ReFS becomes the default only for heavy workloads.

• Most people: nothing to do. If your USB drives use exFAT, FAT32 or NTFS, January changes nothing for them.
• If you use ReFS on an external drive: check Event Viewer for Event ID 524 and move important files to a supported drive before the January 2027 security update.
• If you run Windows for a team: use the audit phase to find affected PCs, then decide on RefsRemovableDeviceMountPolicy through Group Policy. For more Windows changes this month, see our story on the October update's Smart App Control and Wi-Fi issues.