I found and tested this app FolderMount over at the XDA and it works perfectly... i know the rules say dont post links to other websites but for best results and easier support by the Dev im sure it be ok if not take it down..
Mini how to
- Get the apk from link above (requested by Dev)
- Click add to add a folder pair
- Select a source directory that you want to move files from. This should be somewhere in your internal sdcard. Usually located at /mnt/sdcard
- Select a destination directory that you want to move files to. This should be some folder in your external sdcard or OTG drive.
- FolderMount will offer to move the files to the destination. Click yes.
- After the moving process is done, click on the pin. If the pin is green, you're all set.
Note: The files will appear in both the source and destination after pinning them. This is normal as the files that you see in your internal sdcard are actually located on your external sdcard. This is how FolderMount "tricks" apps that don't have the ability to save explicitly to the external sdcard.