Windows write filter
Since the persistent data are stored on a storage medium, the file and the path must be writeable. If you use the Windows write filter, the Windows partition is protected against write access operations, and the persistent data are not saved.
UWF exception list
By default an exception list is automatically created when the UWF is switched on. The \Boot directory is already entered in this list. Check the configuration of the UWF if you have made changes to the exception list.
By default, the persistent data are stored under \TwinCAT\3.1\Boot in TwinCAT 3.
The UWF can be configured via the Beckhoff Unified Write Filter Manager.