There seems to be quite a bit of discussion about this but no real resolution. I've been trying to find a way to mount directories on my external SD card onto the internal /sdcard directories to force apps that only can write data to /sdcard internally (that is, they have that directory path hardwired) to actually write to directories created on the external card (e.g. /storage/xxxx-xxxx/whatever). What a mess. I've tried symbolic links, battled the FUSE mounts, experimented with "mount -o bind" -- on and on. FolderMount claims to do such things, but bricks some phones. Adaptable storage doesn't solve this problem and creates new ones of its own in terms of control.
I can do the bind mounts -- and when I look at them with adb they appear to be correct. But the apps continue to write to the internal /sdcard dirs. I've also turned off SuperSu mount namespace separation in testing.
It's a shame that something that really should be so simple is such a PITA. Any new ideas? Thanks!