ANONYMOUS wrote: I can see that the outcomes should be the same - you make the file copies, and then extend a dynamic data-structure to remember what has been performed. In contrast, the sample solution builds its dynamic data-structure to define what it will copy, and then performs all the activities recorded in it structure. The sample solution successfully recursively syncs directories and their (embedded) child directories *because* it works out what to copy before it starts copying, and does not continue 'processing' the newly created directories.