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androidxamarin.formsstorageunauthorizedusb-otg

Xamarin.Forms Android cannot access USB OTG folder

I'm on an Android 5.0 device where a usb mass storage is mounted at "/storage/usbotg" via USB OTG cable. Unfortunately from my app I only have read access to that folder. Please note that I set the write external storage permission as I'm able to write to device storage.

I post the following code as reference:

string Root = "/storage/usbotg";                        
string[] Dirs = Directory.GetDirectories(Root);

string NewFolder = Path.Combine(Root, "NewFolder");          
Directory.CreateDirectory(NewFolder);

This gives me an exception on the last line (but I'm able to list subdirectories in Dirs)

Exception: System.UnauthorizedAccessException

Exception Message: Access to the path "/storage/usbotg/NewFolder" is denied.

If I use:

string Root =  "/storage/emulated/0";

everything is working fine and the "NewFolder" is created.

What I'm missing? How can I write to that folder?

I'm using Xamarin.Forms 2.5.0

Thanks for your help

Solution

  • Accessing data outside your application's private storage using the file system is more restricted with each Android version.

    The recommended options are :

    • Use getExternalFilesDirs(), getExternalCacheDirs, ... : this gives you one or more directories specific to you application (usually directories named after the package name). This does not work for removable media, unless they're adopted.

    • Use the Storage Access Framework : ask the user (using ACTION_OPEN_DOCUMENT_TREE) to choose the storage root. You can then manage the content of the picked directory through the SAF API. You can persist the permission, so you only need to ask the user once. It seems that it is what ES File explore does to get write permission.

    A (much) more detailed explanation by Mark Murphy.