This setting is found in the "Preferences..." dialog and can be accessed by going to menu: Setup / Preferences..., and select tab "Misc". Scroll down to the option: "Perform File Buffering for Unicode/Network Access".
When checked ACE 3000 will export all files to a temporary folder, and then use the Operating System (OS) Copy command to move the file to the final destination. This method is preferred when exporting files to a remote server, another drive, cloud storage (like dropbox), or if the folder contains Unicode characters.
When unchecked, ACE 3000 writes exported files directly to the destination folder you have selected using the C++ CFile class.
Note: When unchecked recommend to always save to your local HDD (same drive as ACE 3000 is installed).