Have root & external sdcard mounted rw, but mv produced "Permission Denied"

Question

I have an old tablet running LNOS 14.1. It has an external SDCard with some Books, Videos etc. The card is configured as movable storage. .android_secure is an empty directory, and though I haven't deliberately installed any apps on the card, it does appear there are some in the Android directory. I have noticed that though I have root on the tablet, there are things that I can not do via a shell or by ssh'ing in. I can't look in .androd_secure for instance, but more importantly, though I can copy to the root of the SDCard, I can not copy to any of the subfolders! I am able to enter and read from the subfolders, as can apps like Kodi and Librara.

I am sure I have root as I set access permission in Developer tools, and multiple apps have caused the OS to produce pop ups requesting root access. Also, the Root Verifier app reports that I have root. In addition, Termux and simpleSSHD allow me to su - root, & I am able to do some things with root that I can't otherwise. eg. without root I can't write to the SDCard at all! But I still can't do this.

A friend suggested SELinux issues, so I include ls -Z. Is there a way around this?

---

# whoami
root

# pwd
/storage/1dc31997-9ebf-4573-9c75-3e57793b4071

# cat /proc/mounts | grep "/storage/1"
/dev/fuse /storage/1dc31997-9ebf-4573-9c75-3e57793b4071 fuse rw,nosuid,nodev,noexec,noatime,user_id=1023,group_id=1023,default_permissions,allow_other 0 0

# ls -la
ls: ./.android_secure: Permission denied
total 5852
drwxrwx--x 12 root sdcard_rw    4096 2020-12-12 01:53 .
drwxr-xr-x  5 root root          100 2020-12-11 18:49 ..
drwxrwx--x  5 root sdcard_rw    4096 2020-12-09 22:56 .Trash-1000
drwxrwx--x  2 root sdcard_rw    4096 2020-12-10 22:49 AOSP.7.0.Root.Install.amaces
drwxrwx--x  5 root sdcard_rw    4096 2020-12-09 22:04 Android
drwxrwx--x  2 root sdcard_rw    4096 2020-12-10 04:38 Apps
drwxrwx--x 25 root sdcard_rw    4096 2020-12-06 19:04 Books
drwxrwx--x  2 root sdcard_rw    4096 2020-12-10 22:45 LNOS.14.1.amaces
drwxrwx--x  2 root sdcard_rw    4096 2000-01-02 12:25 LOST.DIR
drwxrwx--x  3 root sdcard_rw    4096 2020-10-25 02:18 TWRP
drwxrwx--x  3 root sdcard_rw    4096 2020-10-29 23:41 Videos
-rw-rw----  1 root sdcard_rw    5294 2020-12-12 01:53 bottled.zip
-rw-rw----  1 root sdcard_rw 5888840 2020-12-11 04:49 org.fdroid.fdroid.privileged.ota_2110.zip

# cp bottled.zip bottled.1.zip

# ls -lZ
ls: ./.android_secure: Permission denied
total 5848
drwxrwx--x  2 root sdcard_rw u:object_r:fuse:s0    4096 2020-12-10 22:49 AOSP.7.0.Root.Install.amaces
drwxrwx--x  5 root sdcard_rw u:object_r:fuse:s0    4096 2020-12-09 22:04 Android
drwxrwx--x  2 root sdcard_rw u:object_r:fuse:s0    4096 2020-12-10 04:38 Apps
drwxrwx--x 25 root sdcard_rw u:object_r:fuse:s0    4096 2020-12-06 19:04 Books
drwxrwx--x  2 root sdcard_rw u:object_r:fuse:s0    4096 2020-12-10 22:45 LNOS.14.1.amaces
drwxrwx--x  2 root sdcard_rw u:object_r:fuse:s0    4096 2000-01-02 12:25 LOST.DIR
drwxrwx--x  3 root sdcard_rw u:object_r:fuse:s0    4096 2020-10-25 02:18 TWRP
drwxrwx--x  3 root sdcard_rw u:object_r:fuse:s0    4096 2020-10-29 23:41 Videos
-rw-rw----  1 root sdcard_rw u:object_r:fuse:s0    5294 2020-12-12 03:26 bottled.1.zip
-rw-rw----  1 root sdcard_rw u:object_r:fuse:s0    5294 2020-12-12 01:53 bottled.zip
-rw-rw----  1 root sdcard_rw u:object_r:fuse:s0 5888840 2020-12-11 04:49 org.fdroid.fdroid.privileged.ota_2110.zip

# mv bottled.zip Videos
mv: bad 'bottled.zip': Permission denied

# cd Videos

# ls -lZ                                                                                                                                             
total 7434136
-rw-rw---- 1 root sdcard_rw u:object_r:fuse:s0  402069980 2015-07-24 03:56 Hold Fast-15351476.mp4

# mv ../bottled.zip .
mv: bad '../bottled.zip': Permission denied

Answer 1

@IrfanLatif offered a comment that included an excellent link to all the things root can't do. It turns out, reading a FUSE filesystem owned by another user, is one of them. Android, will mount a device at multiple places, each with different permissions. Going to /storage/[UUID], you are visiting a FUSE filesystem. On the other hand, going to /mnt/media_rw/[UUID], you are visiting the filesystem directly. There may, in addition, be SELinux permission issues, as described in that link, but in my case those did not appear.

$ mount | grep 1dc31997
/dev/block/vold/public:179_97 on /mnt/media_rw/1dc31997-9ebf-4573-9c75-3e57793b4071 type f2fs (rw,dirsync,context=u:object_r:sdcard_posix:s0,nosuid,nodev,noatime,background_gc=on,user_xattr,active_logs=6)
/dev/fuse on /mnt/runtime/default/1dc31997-9ebf-4573-9c75-3e57793b4071 type fuse (rw,nosuid,nodev,noexec,noatime,user_id=1023,group_id=1023,default_permissions,allow_other)
/dev/fuse on /mnt/runtime/read/1dc31997-9ebf-4573-9c75-3e57793b4071 type fuse (rw,nosuid,nodev,noexec,noatime,user_id=1023,group_id=1023,default_permissions,allow_other)
/dev/fuse on /mnt/runtime/write/1dc31997-9ebf-4573-9c75-3e57793b4071 type fuse (rw,nosuid,nodev,noexec,noatime,user_id=1023,group_id=1023,default_permissions,allow_other)
/dev/fuse on /storage/1dc31997-9ebf-4573-9c75-3e57793b4071 type fuse (rw,nosuid,nodev,noexec,noatime,user_id=1023,group_id=1023,default_permissions,allow_other)