[JiaYu S3 Questions & Answers] pfavayi:
I had a problem writing to external storage (Micro SD card) on madOS (android 7.1.2)

After an install I just could not write to the SD card , even after root ...

And this is a problem that affects android from marshmallow onwards (android 5.1 and later)

So here is how I solved the problem...

1) I downloaded on my computer and then transfered to the SD card the following files:

1-- a recovery image of madOS (android 7.1.2) I got from here

https://xdaforums.com/devdb/project/?id=16842#downloads

2-- the pico package of opengapps from here

http://opengapps.org/

(choose your platform , your android version and the pico variant)

3-- magisk manager (an apk ) and the magisk zip from

https://xdaforums.com/apps/magisk/official-magisk-v7-universal-systemless-t3473445

magisk roots like SuperSU , but its the new form of root called systemless root and to pokemo go , safetyNet etc , it appears like your phone is not root and you can use them - but you can do much more with it than the other root applications !!

2) I inserted the SD card into the phone (a jiayu s3 ) and booted into recovery (by pressing the vol up + power button untill the phone switches on and then releasing the power button but keep holding the vol up

I already have twrp 3.1.1-0 installed (madOS requires its own version of twrp otherwise you can't install it. You can download their twrp from here - scroll down on the page to find the link https://xdaforums.com/jiayu-s3/development/rom-unofficial-aosp-n-jiayu-s3-t3451923 )

3) I wiped everything except system and external storage (I wanted a clean install !!)

4) I installed madOS

5) Without doing a wipe I intalled opengapps then wiped cache and delvicahe and rebooted into system

6) I did a few settings on the phone then shutdown the phone and booted into recovery again

7) I installed magisk zip file , wiped cache and delvic cache , and rebooted into the system

8) Now I was rooted (by magisk) - a systemless root , which means there is nothing writen to /system and so it looks like the phone is not rooted to applications that check root !!

9) I installed magisk manager apk , loaded the app , saw that I was properly rooted (be connected to the internet), clicked the downloads button , and clicked to install "App Systemizer". After the install you get a "AppSystemizer" app

I then installed the apps I want from google play store

What App Systemizer does is make a user installed app behave like it was a system app.

SYSTEM APPS CAN WRITE TO EXTERNAL STORAGE SD CARD !!! because they are trusted by the system !!

So I opened AppSystemizer and choose ES file explorer to make it a system app ( when you save you have to reboot phone) , and after that I can use ES file explorer on SD card without problems !!

I did the same to some other applications that I wanted to be able to write to the SD card ...

AND THE GOOD THING IS YOU DONT HAVE TO GRANT ROOT TO ANY APPLICATION !!!

I was always afraid to grant root to ES file explorer because while its the best or one of the best file managers , it has many ads , requires too many permissions when it installs , writes some files to /system when you give it root and I beleive its a chineese spy software .

So now I can now use ES file explorer as I want and without root and so without fear !!!

10) Another good app to install is adaway - it stops all those adds from applications , including ES file explorer

https://labs.xda-developers.com/store/app/org.adaway

After this you will have a smooth android experience

I beleive this is applicable to any marshmallow and Nougat rom (stock or custom)