As far as I understand it is possible to write directly to disk on Windows, bypassing the file system, and presumably this is precisely what disk shredder software does in order to shred empty space on disk. Similarly SQL Server probably does direct writing to disk within the confines of its MDF data files.
Now, does Windows OS or any other known major software (other than shredder) write data to disk outside of legit Windows files, e.g. in order to create its own quasi file system in disk sectors which, from the standpoint of Windows file system, are "unused free space"?
I guess I am primarily interested in this with regards to Windows itself because that would have important implications for any work having to do with monitoring or imposing lock down on OS activities.