[Galaxy S 4 Q&A, Help & Troubleshooting] Guestx00425:
Hi folks,

I recently upgraded my rooted Galaxy S4 (I9505) from CM12.1 to CM13, so I now have Marshmallow installed. Before the upgrade I made a backup on my external sd card with TWRP 3.0.2. Upgrading to Marshmallow went smooth and so I made another backup after installing CM13.

Yesterday I tried to make a backup but the backup on the external sd card failed. I tried it at least ten times. Most of the time I had some errors but sometimes there was no error shown in the log but the backup files were not saved on the external sd card. I've attached the complete recovery.log, here is an excerpt from the log:

* MD5 erstellt.
Durchschnittliche Sicherungsgeschwindigkeit für Dateisysteme: 13 MB/sek
Durchschnittliche Sicherungsgeschwindigkeit für abgebildete Laufwerke: 10 MB/sek
Fehler beim Öffnen: '/external_sd/TWRP/BACKUPS/a19a120c/2016-07-27--20-28-45_cm_jfltexx-userdebug_6.0.1_MOB30R_251a6113e//�?�Q?^??.?E�' (
I:Real error: Unable to stat '/external_sd/TWRP/BACKUPS/a19a120c/2016-07-27--20-28-45_cm_jfltexx-userdebug_6.0.1_MOB30R_251a6113e//�?�Q?^?
Fehler beim Öffnen: '/external_sd/TWRP/BACKUPS/a19a120c/2016-07-27--20-28-45_cm_jfltexx-userdebug_6.0.1_MOB30R_251a6113e//"�F?�&EQ.|??' (
I:Real error: Unable to stat '/external_sd/TWRP/BACKUPS/a19a120c/2016-07-27--20-28-45_cm_jfltexx-userdebug_6.0.1_MOB30R_251a6113e//"�F?�&E
Fehler beim Öffnen: '/external_sd/TWRP/BACKUPS/a19a120c/2016-07-27--20-28-45_cm_jfltexx-userdebug_6.0.1_MOB30R_251a6113e//����w?@P.?E)
I:Real error: Unable to stat '/external_sd/TWRP/BACKUPS/a19a120c/2016-07-27--20-28-45_cm_jfltexx-userdebug_6.0.1_MOB30R_251a6113e//����w?@
Fehler beim Öffnen: '/external_sd/TWRP/BACKUPS/a19a120c/2016-07-27--20-28-45_cm_jfltexx-userdebug_6.0.1_MOB30R_251a6113e//?rG?????.\N�' (
I:Real error: Unable to stat '/external_sd/TWRP/BACKUPS/a19a120c/2016-07-27--20-28-45_cm_jfltexx-userdebug_6.0.1_MOB30R_251a6113e//?rG????
Fehler beim Öffnen: '/external_sd/TWRP/BACKUPS/a19a120c/2016-07-27--20-28-45_cm_jfltexx-userdebug_6.0.1_MOB30R_251a6113e//k�?�p?�!.?e<' (
I:Real error: Unable to stat '/external_sd/TWRP/BACKUPS/a19a120c/2016-07-27--20-28-45_cm_jfltexx-userdebug_6.0.1_MOB30R_251a6113e//k�?�p?�
Fehler beim Öffnen: '/external_sd/TWRP/BACKUPS/a19a120c/2016-07-27--20-28-45_cm_jfltexx-userdebug_6.0.1_MOB30R_251a6113e//-)

"Fehler beim Öffnen" is German for I guess "Error while opening…"

The sd card works in the way that I can put photos and music on it. It is mounted as follows:

root@jfltexx:/mnt/sdcard # mount | grep 2015-1707
/dev/block/vold/public:179_33 on /mnt/media_rw/2015-1707 type vfat (rw,dirsync,nosuid,nodev,noexec,relatime,uid=1023,gid=1023,fmask=0007,dmask=0007,allow_utime=0020,codepage=cp437,iocharset=iso8859-1,shortname=mixed,utf8,errors=remount-ro)
/dev/fuse on /mnt/runtime/default/2015-1707 type fuse (rw,nosuid,nodev,noexec,noatime,user_id=1023,group_id=1023,default_permissions,allow_other)
/dev/fuse on /mnt/runtime/read/2015-1707 type fuse (rw,nosuid,nodev,noexec,noatime,user_id=1023,group_id=1023,default_permissions,allow_other)
/dev/fuse on /mnt/runtime/write/2015-1707 type fuse (rw,nosuid,nodev,noexec,noatime,user_id=1023,group_id=1023,default_permissions,allow_other)
/dev/fuse on /storage/2015-1707 type fuse (rw,nosuid,nodev,noexec,noatime,user_id=1023,group_id=1023,default_permissions,allow_other

Yesterday I tried to change permissions on the sd card by setting write permissions like this:

root@jfltexx:/mnt # chmod g +w media_rw/

Nothing changed.

Furthermore I tried the app "SDFix: KitKat Writable MicroSD" to fix the permission problem. If I had read the description more carefully I would have saved some time. The app had no effect and said that it couldn't fix the permission problem so I uninstalled it immediately. Today I tried the Xposed module "Marshmallow SD fix" and gave permission for Nandroid Manager but that didn't help either. But that was what I expected because I think (!) that this has nothing to do with TWRP.

So my question is: What do I have to do to make TWRP backup my data on the external sc card?

Your help is very much appreciated. If you need further information just let me know.

Greeting, Jens