How to save files to external SD card on a non-rooted Android?

I'm using the Terminal Emulator app on a non-rooted Android 5.0.2 phone.

When moving files from a folder of the external SD card to another with:

mv /storage/extSdCard/DCIM/Camera/* /storage/extSdCard/phone/

I get:

mv: rename /storage/extSdCard/DCIM/Camera/20170519_161315.jpg to 
     /storage/extSdCard/phone/20170519_161315.jpg: Permission denied

Is there a solution for this (without rooting the phone)?

---

Note: ls -la in /storage/extSdCard gives:

drwxrwx--x root     sdcard_r          2016-08-10 02:18 Android
drwxrwx--- root     sdcard_r          2016-11-20 09:45 DCIM
...
drwxrwx--- root     sdcard_r          2017-05-19 15:46 phone