A little old, but it can be useful to someone

Root Android 4.4 + !!!

Make a backup "platform.xml"

1. Launch your favorite file manager with enabled root access rights.
You can use any app you prefer which can run elevated on your device.
For example, the 'Root Explorer' app or 'Total Commander' or 'File Manager' .

2. Navigate to the following file:

/system/etc/permissions/platform.xml

3. Find android.permission.WRITE_EXTERNAL_STORAGE and android.permission.WRITE_MEDIA_STORAGE lines. These are XML sections. You need to make them look exactly like the strings below:

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

Save the platform.xml file. It’s necessary to set the file permissions to 644 (rw-/r–/r–) before mobile restarting. Now reboot your Android device.