There is no generalized way to access the free space of a mounted and mutable file system. What you'd be better off doing on Unix type OSes is creating a large

There is no generalized way to access the free space of a mounted and mutable file system. What you'd be better off doing on Unix type OSes is creating a large empty file, deleting it but keeping the file handle, and reading/writing that. That's a traditional way to make a self erasing scratch file. It doesn't work on Windows, though, because Windows won't let you delete a file that's open.