UnifiedWriteFilter (reference)

Use UnifiedWriteFilter to configure settings for the Unified Write Filter (UWF) in your device to help protect your physical storage media, including most standard writable storage types that are supported by the OS, such as physical hard disks, solidate-state drives, internal USB devices, external SATA devices, and so on. You can also use UWF to make read-only media appear to the OS as a writeable volume.


You cannot use UWF to protect external USB devices or flash drives.

UWF intercepts all write attempts to a protected volume and redirects those write attempts to a virtual overlay. This improves the reliability and stability of your device and reduces the wear on write-sensitive media, such as flash memory media like solid-state drives.

The overlay does not mirror the entire volume, but dynamically grows to keep track of redirected writes. Generally the overlay is stored in system memory, although you can cache a portion of the overlay on a physical volume.


UWF fully supports the NTFS system; however, during device startup, NTFS file system journal files can write to a protected volume before UWF has loaded and started protecting the volume.

Learn more about the Unified Write Filter feature.

Applies to

Setting Desktop editions Mobile editions Surface Hub HoloLens IoT Core
All settings X X


Set to True to enable UWF.


OverlayFlags specifies whether to allow writes to unused space on the volume to pass through, and not be redirected to the overlay file. Enabling this setting helps conserve space on the overlay file.

  • Value 0 (default value when OverlayType is not Disk): writes are redirected to the overlay file
  • Value 1(default value when OverlayType is Disk): writes to unused space on the volume are allowed to pass through without being redirected to the overlay file.


Enter the maximum overlay size, in megabytes (MB), for the UWF overlay. The minimum value for maximum overlay size is 1024.


UnifiedWriteFilter must be enabled for this setting to work.


OverlayType specifies where the overlay is stored. Select between RAM (default) and Disk (pre-allocated file on the system volume).


You can add or remove registry entries that will be excluded from UWF filtering. When a registry key is in the exclusion list, all writes to that registry key bypass UWF filtering and are written directly to the registry and persist after the device restarts.

Use Add to add a registry entry to the exclusion list after you restart the device.

Use Remove to remove a registry entry from the exclusion list after you restart the device.


Set to True to reset UWF settings to the original state that was captured at installation time.


Enter a drive letter for a volume to be protected by UWF.


In the current OS release, Windows Configuration Designer contains a validation bug. To work around this issue, you must include a ":" after the drive letter when specifying the value for the setting. For example, if you are specifying the C drive, you must set DriveLetter to "C:" instead of just "C".