Windows 11 will stop mounting ReFS USB drives by default once Microsoft’s January 2027 security update arrives, according to Microsoft’s own support page. Microsoft describes the change as a refusal to mount the volume, not as erasing it, but a drive that will not open is a problem all the same. Most flash drives are formatted as exFAT, FAT32 or NTFS and will never notice. If you or your workplace ever set up an external disk with ReFS, the warning phase is already running, and now is the cheap time to deal with it.
TL;DR: Windows 11 will refuse to mount ReFS volumes on USB drives and other external disks once Microsoft’s January 2027 security update lands. A warning phase began with the October 13, 2026 update. The block targets ReFS volumes only, so check any drive you formatted that way and copy its data to another file system while Windows can still open it.
What Microsoft is changing for ReFS USB drives in Windows 11
Microsoft’s document, published October 13, 2026, says ReFS on removable media, such as a USB disk, is “not a supported configuration.” The company is rolling the change out in two phases, and the first is already live.
| Phase | Update | What Windows does | Default |
|---|---|---|---|
| Audit | Security update released October 13, 2026 | Logs a warning for each affected volume | Auditing on, blocking off |
| Enforcement | January 2027 security update | Refuses to mount affected ReFS volumes | Auditing and blocking on |
Microsoft has not given an exact January date. Security updates normally arrive on Patch Tuesday, which would be January 12, 2027, though that is my inference and not something the page states.
The group most likely to be caught is a smaller one than the headline suggests.
Which drives the Windows 11 block covers
The page lists Windows 11 versions 22H2, 23H2, 24H2, 25H2 and 26H1, plus Windows 10 22H2 and several Windows Server releases. The list stops at 26H1, so I would treat a PC that has just moved to Windows 11 26H2 as covered too, although the page does not say so.
The wording covers “removable or externally attached” disks, so an external hard drive formatted as ReFS falls inside it, not only a flash drive. Microsoft says the audit shows up as Event ID 524, source ReFS, in the System log, and warns that the volume would not mount if blocking were on.
For most people this will be invisible until the one morning it isn’t. The old backup drive in a desk drawer is the likeliest casualty.
Administrators get a switch to control all of this.
The setting administrators can change
Microsoft names the setting RefsRemovableDeviceMountPolicy, a DWORD with 0 for off, 1 for audit and 2 for block. Audit is the default today.
The Group Policy copy sits under HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Policies and the local copy under HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\FileSystem. Group Policy wins if both are set, and a restart is needed after any change.
Microsoft’s own advice is to test block mode in a controlled environment before turning it on widely. Home users have no such buffer, which is why checking your drives yourself matters.
How to check your drives before January
Plug the drive in, right-click it in File Explorer and choose Properties. The File system line will read ReFS, NTFS, exFAT or FAT32. Microsoft is testing a redesigned right-click menu in the Beta Channel, so the menu may look different on some builds.
If it says ReFS, copy the data to a drive formatted with another file system before January, while Windows can still open it. The page tells you to migrate required data to a supported storage configuration and does not name a format. NTFS and exFAT are the usual choices for USB storage.
Do the copy first and reformat afterward, since formatting erases what is on the drive.
