Here's my latest find.

Please excuse the direct copy and paste but I wanted to save this for my own notes.
This post is a direct lift from here ...

http://zidroid.com/how-to-fix-extsdcard-mount-issue-on-kitkat-and-lollipop/

Big, big thanks to ZiDroid for helping me with my write to SD card problem.

How to Fix ExtSdCard Mount Issue on KitKat and Lollipop

Since Android 4.4.2 KitKat was released to the Galaxy S4 / Note 3 and other Android devices which support external SD card, more users were having issue with some third-party apps in saving, writing or copying  files to the external SD card due to KitKat 4.4 security which was added by Google.

This is how to fix ExtSdCard Issue on Android KitKat, Lollipop and later for all android devices …

Requirements :

• Rooted Android device running on Android KitKat or Lollipop update.
• Installed ES File Manager or any root files explorer app.

Steps :

Learn how to put Root access on your phone, and do so.
See my post on How do I root My Phone at this link.

Open “ES File Explorer” and press on the top left icon.
Go to Tool > Root Explorer > Mount R/W

Change Path “/” and “/system” from RO to RW.

Go to Local > / Device > system > etc > permissions > and open platform.xml as text in “ES Note Editor”.

Find the following line:

“android.permission.WRITE_EXTERNAL_STORAGE” permission

Change it from :

permission name="android.permission.WRITE_EXTERNAL_STORAGE"
group gid="sdcard_r"
group gid="sdcard_rw"
/permission

To

permission name="android.permission.WRITE_EXTERNAL_STORAGE"
group gid="sdcard_r"
group gid="sdcard_rw"
group gid="media_rw"
/permission

See this pic from ZiDroid that shows the exact format with brackets etc.

Save the changes to the file.

Check the file again and make sure that everything is OK.
After rebooting you will be able to write on the external SD card without any problem.

This has worked for me on my Huawei Mate 7. I like to use the Open Camera app but Android 4.4.2 wouldn't let this app save photos to the external SD card. After this fix, I can now save to the SD card.
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