is it only path in /etc/fmdita/dita_resources? Does it work for other paths?
Checked drag and drop from local drive to mount drive on MAC for a different path on my 6.4 instance and was able to add multiple files to AEM through webdav
Another way you could add the resources is through code, which is suitable if its a one time job and build the application package as part of maven project
Well there must be some kind of information [logs] on the webdav client if uploads are not working for certain paths. Please check if there's a difference for the path where upload works and doesn't. It might be worth to try a different webdav client