I am currently working on a module which allows you to easily mount internal SD card folders to external SD folders. This module installs a script in /system/bin, so you can use it later in your scripts, flows or even apps.
Currently the script allows you to mount and unmount folders. Later, in v0.9 these folders will be added to a list, which will rebind all currently mounted folders after a reboot.
The basic usage for mounting a folder:
fmount -b [source] (destination)
fmount -b "WhatsApp" ".foldermount/WhatsApp"
To unmount a folder, just enter the following command:
fmount -u [source]
This module aims to be a alternative solution to FolderMount, a app created by madmack, but unfortunately his app does not work with MagiskSU anymore. With v0.9 it is planned to add the ability to rebind folders at post-fs-data, so these will be still mounted even after a reboot.
For more information, see this issue on GitHub.
Magisk FolderMount, Tool/Utility for the XDA Community Apps
Source Code: https://github.com/codebucketdev/magisk-foldermount
Current Beta Version: v0.8.5
Beta Release Date: 2017-04-18
Last Updated 2017-04-18