external sd – Mounting portable SD card folder to internal folder (WhatsApp)

I am trying to mount WhatsApp media directory from portable SD to internal “SD”, as the phone internal storage is very limited.

My phone:
Sony Xperia XZ1 Compact (32GB internal storage, 128GB external SD card), Pie 9.0, stock image, bootloader unlocked, dm-verity, and SElinux enforcing. Magisk 20.1 (mount Namespace Mode= Global Namespace). bindfs v.1.14.2.

I have read and tried what is shown in these threads:

Output of /proc/filesystems

nodev   sysfs
nodev   rootfs
nodev   ramfs
nodev   bdev
nodev   proc
nodev   cpuset
nodev   cgroup
nodev   cgroup2
nodev   tmpfs
nodev   configfs
nodev   debugfs
nodev   tracefs
nodev   sockfs
nodev   pipefs
nodev   devpts
nodev   ecryptfs
nodev   sdcardfs
nodev   fuse
nodev   fusectl
nodev   pstore
nodev   selinuxfs
nodev   functionfs

The last relevant lines of output from mount -r

/data/media on /mnt/runtime/default/emulated type sdcardfs (rw,nosuid,nodev,noexec,noatime,fsuid=1023,fsgid=1023,gid=1015,multiuser,mask=6,derive_gid,default_normal)
/data/media on /storage/emulated type sdcardfs (rw,nosuid,nodev,noexec,noatime,fsuid=1023,fsgid=1023,gid=1015,multiuser,mask=6,derive_gid,default_normal)
/data/media on /mnt/runtime/read/emulated type sdcardfs (rw,nosuid,nodev,noexec,noatime,fsuid=1023,fsgid=1023,gid=9997,multiuser,mask=23,derive_gid,default_normal)
/data/media on /mnt/runtime/write/emulated type sdcardfs (rw,nosuid,nodev,noexec,noatime,fsuid=1023,fsgid=1023,gid=9997,multiuser,mask=7,derive_gid,default_normal)
/dev/block/sda60 on /mnt/qns type ext4 (rw,seclabel,nosuid,nodev,noexec,noatime,discard,data=ordered)
/dev/block/vold/public:179,1 on /mnt/media_rw/6534-3933 type texfat (rw,dirsync,nosuid,nodev,noexec,noatime,uid=1023,gid=1023,umask=0007,allow_utime=0020,utf8,min_prealloc_size=64k,max_prealloc_size=121910k,writeback_boundary=4M,readahead=4M,fail_safe,discard,hidden=show,errors=continue)
/mnt/media_rw/6534-3933 on /mnt/runtime/default/6534-3933 type sdcardfs (rw,nosuid,nodev,noexec,noatime,fsuid=1023,fsgid=1023,gid=1015,mask=6,nocache)
/mnt/media_rw/6534-3933 on /storage/6534-3933 type sdcardfs (rw,nosuid,nodev,noexec,noatime,fsuid=1023,fsgid=1023,gid=1015,mask=6,nocache)
/mnt/media_rw/6534-3933 on /mnt/runtime/read/6534-3933 type sdcardfs (rw,nosuid,nodev,noexec,noatime,fsuid=1023,fsgid=1023,gid=9997,mask=18,nocache)
/mnt/media_rw/6534-3933 on /mnt/runtime/write/6534-3933 type sdcardfs (rw,nosuid,nodev,noexec,noatime,fsuid=1023,fsgid=1023,gid=9997,mask=18,nocache)

The output of proposed command:

bindfs -u u0_a195 -g 9997 -p a-rwx,ug+rw,ug+X /mnt/media_rw/6534-3933/WhatsApp/Media /mnt/runtime/write/emulated/0/WhatsApp/Media

Gives me the following error:

fuse: mountpoint is not empty
fuse: if you are sure this is safe, use the 'nonempty' mount option

I then add the nonempty string to the command, and then the external SD card directory mounts as “internal”.

The new lines from mount -r now reads:

/mnt/media_rw/6534-3933/WhatsApp/Media on /mnt/runtime/write/emulated/0/WhatsApp/Media type fuse (rw,nosuid,nodev,relatime,user_id=0,group_id=0,default_permissions,allow_other)
/mnt/media_rw/6534-3933/WhatsApp/Media on /storage/emulated/0/WhatsApp/Media type fuse (rw,nosuid,nodev,relatime,user_id=0,group_id=0,default_permissions,allow_other)
/mnt/media_rw/6534-3933/WhatsApp/Media on /mnt/runtime/default/emulated/0/WhatsApp/Media type fuse (rw,nosuid,nodev,relatime,user_id=0,group_id=0,default_permissions,allow_other)
/mnt/media_rw/6534-3933/WhatsApp/Media on /mnt/runtime/read/emulated/0/WhatsApp/Media type fuse (rw,nosuid,nodev,relatime,user_id=0,group_id=0,default_permissions,allow_other)

WhatsApp does READ all the files correctly, however I cannot download anything new to the drive. This is frustrating as hell, as I have come to this stage before.
I just can seem to find out how to make WhatsApp being able to WRITE to the directory.

I have used apps like apps2sd, ts-bind, fbind, mount options. Anything under the sun. I can never reach a stage longer than WhatsApp being “tricked” to read from external SD, but never being able to write to external SD.

The only working solution, is not a viable solution, is to install experimental xposed, install old module xinternalSD, and magisk module “ExSDCard Access Enabler”. Then WhatsApp works perfectly, but I believe this is a very unstable for the rest of my system and my phone. And I would really like to have a more clean solution to this problem, like the one you are proposing.

Please assist in this great great headache.

Last note, also tried the command as given from your other post:

bindfs -o nosuid,nodev,noexec,noatime,context=u:object_r:sdcardfs:s0 -u 0 -g 9997 -p a-rwx,ug+rw,ugo+X --create-with-perms=a-rwx,ug+rw,ugo+X --xattr-none --chown-ignore --chgrp-ignore --chmod-ignore /mnt/media_rw/6534-3933/WhatsApp/Media /mnt/runtime/write/emulated/0/WhatsApp/Media

Same result after adding nonempty option, WhatsApp can read from external SD, but cannot write to it.