See this forum thread for details, but I'm frequently getting 'FileOpenError' with message Must be unbuffered on Windows with 5.x and 6.0 versions.

I think I was able to track it down to incrementalDeleteHelper() in the IAsyncFile actor that is called to delete old storage or log files. It attempts to get the size of files it is removing, by opening them. In this instance, it does not pass the OPEN_UNBUFFERED flag when calling open(), which was causing the Windows port to throw in 5.x/6.0. This was changed to an ASSERT() in both windows and linux ports recently.