android mkdirs returns false on sd card

Question

I made a little file explorer in my app and I want to support creating new folders.

When navigating to a location in the device's built in memory, everything works as expected but when I navigate to my sd card and try to create a new folder, the line new File(path + File.separator + newFolderName).mkdirs() returns false.

I've made the following debug tests:

String path = "/storage/external_SD";   // Not hard coded - user navigates here
new File(path).isDirectory();            --> true
new File(path).canRead();                --> true
new File(path).canWrite();               --> true

String newFolderPath = path + File.separator + newFolderName;
new File(newFolderPath).isDirectory();   --> false
new File(newFolderPath).mkdir();         --> false
new File(newFolderPath).mkdirs();        --> false

Something odd I've noticed: I put a breakpoint in the File class on the first line of mkdirs() but the application didn't stop on the line and immediately returned false. This made me think that's a permissions issue even though I have the correct permission in the right location:

<manifest ...>

    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

    <application ...>
    ...
    </application>

</manifest>

What am I missing?

Answer 1

Since Android 5.0, an application may only write to it's private location on an external SD-card.

For all other locations, it needs to implement a document provider.

The strategy to access external storage is outlined in this Google documentation.