Default platform.xml

I'm new in XDA and new with Android (and almost new with the English language . Maybe this is the wrong thread, if so, please help to find the right one. I fight with the following Problem (HTC One M8, rooted): I try to add the additional permission for WRITE_EXTERNAL_STORAGE in order to get better access to the external SDCARD (system\etc\permissions). I can do this with either the editor or the SDFix app. Everything looks fine, i.e. if I edit the file later with another editor, I see the additional line and the modification-timestamp of the file reflects my amendment. BUT: After reboot this file has returned to its initial state, i.e. really the old file with the original (old) timestamp. What do I wrong?? Besides another question: where can I find a detailed description about the permission logic (app - storage - file). Thanks a lot for any help. Bruno