sblisesivdin, I note you say "mostly accessed for writing", if that "mostly" means you also need to be aware of another process reading, its a bit hard. Reading won't change the hash for example.
Important question - what operating system are you using ? On linux, you can check the atime of all the files, would be a very fast process, that would tell you if the files had been written to or read. On almost all systems, atime is enabled by default. I expect (but don't know for sure) that MacOS will also provide atime.
I am no windows expert but I believe NTFS does not offer atime. Almost impossible to tell if the file has been read. I think....
Davo