Create folder in SD Card (other external storage device)

Question

I try to create a folder in SD Card (other than device memory) to backup some files. To identify storage devices I use...

    List<String> allStoragePlaces = new ArrayList<>();
    File[] listExternalDirs = ContextCompat.getExternalFilesDirs(getApplicationContext(), null);

    for (int i = 0; i < listExternalDirs.length; i++) {
        if(listExternalDirs[i] != null) {
            String path = listExternalDirs[i].getAbsolutePath();
            int indexMountRoot = path.indexOf("/Android/data/");
            if(indexMountRoot >= 0 && indexMountRoot <= path.length()){
                allStoragePlaces.add( i, path.substring(0, indexMountRoot) );
            }
        }
    }

In my case this producess...

• 0) /storage/emulated/0
• 1) /storage/4960-1EFE

I try to generate folder "bak" in /storage/4960-1EFE with no sucess with

File bakDirExtern = new File( "/storage/4960-1EFE/bak");
if( ! bakDirExtern.exists() || ! bakDirExtern.isDirectory()) {
    if(!bakDirExtern.mkdirs()) {
        Log.v("wew", "FOLDER NOT GENERATED");
    }
}