Essentially, unless you `fsync`, there's no guarantee that your data will be durably written to disk. This is because the operating system keeps data buffered in in-memory caches, so if the machine crashes you may lose some data. The `fsync` system call forces the data to be flushed from the in-memory OS cache to the disk. As far as I could tell, the Flush you use does not `fsync`.

Thanks for your input @sakras. I'll fix this