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I just try to ask this question over here, nobody apparently is able to answer this question:

On Android (>4.4) the SD card is not writeable, except for the Android/data/[package-url]/ directory. The solution to this is to root your phone and to edit a config. The phone is an LG Stylus2 and has Andoid 6.0.1. The phone does not allow to format the SD card as internal storage (which would resolve the problem)

BUT: when I install e.g. a file manager like "Simple File Manager" which is a normal apk like all others and installed on a non-rooted phone, the SFM has read/write access to the entire SD card. I asked the author of that package why SFM has R/W access to the entire SD card and his answer was:

Well the app uses the casual Storage Access Framework, thats the way SD card files have to be handled by all apps.

Nobody seems to know the answer to this question. Someone over here maybe?

Richard.

P.S.: e.g. the F-droid app "syncopoli" has WRITE_EXTERNAL_STORAGE set and has no RW permission to SD card, except for its own dir. Remote to local works btw contrary to what the site says.

P.S.2: rsync wrapper even does not even propose dirs outside Android/data/[package-url]/ directory.

P.S. 3: I'm not the only one

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    Unclear if you are asking about writing an app as it is off-topic here. If you are a developer (or trying to think like one) - Commonsware has posted numerous times on the subject of Android storage along with what is to come with Q – Morrison Chang Apr 23 at 17:37
  • No, I'm not writing an app. It is just two apps on a non-rooted phone, rules say the write permissions are restricted for apps, but apparently apps can have full R/W permissions, but nobody can tell me why. Even the author of the app that has full R/W permissions on SD card cannot tell me why his app has full R/W permissions. – Richard Lucassen Apr 23 at 19:48
  • The storage rules for Android has changed from version to version. Enforcement of permissions can vary due to the target API that APK is built with and on which version the OS is running. It may help to edit your question and focus on one app (preferred open-source) and specify exactly which make/model/version of Android you are running. I.e. depending on which "Simple File Manager" you installed, it may be targeting older <4.4 APIs which allow permissive behavior. – Morrison Chang Apr 23 at 20:03
  • As I noted in the question: the app that has R/W permissions is "Simple File Manager" and one of the apps that should have R/W permissions is "syncopoli". Note that syncopoli is only available in the F-droid repository. The Simple File Manager is available in Play store and F-droid repository. – Richard Lucassen Apr 23 at 21:12
  • Yes, but edit your question with links to the version you are using, along with what make (Samsung/LG/Hwawei/Oppo/etc.) and model (Galaxy J/G7/M5 etc.) as manufacturers may do something different to expectations. – Morrison Chang Apr 23 at 22:04

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