Foobar2000 Mobile:Android folder picker
Android folder picker readme
Resort to the modern picker only when a folder cannot be added using the regular one, or regular one is giving you read-only access while you require read-write access instead.
Due to the crazy mess that Android's mass storage concepts are, you will encounter to distinct folder picker dialogs in foobar2000 mobile when running on a modern Android device.
The "old" picker - implemented by foobar2000 - uses classic POSIX interface to walk device's filesystem. Depending on your device maker's decisions it might not let you access specific devices - USB storage in particular. Also, certain devices - typically removable SD cards - will turn up read-only.
The "modern" picker is implemented by the Android system and available on Android versions 5 (Lollipop) and up; it uses DocumentFile interface and none of the above limitations - it will allow you to pick any properly mounted storage device and obtain write access rights to it. The catch is that folders handed to us this way are ridiculously slow to index for music files - you should avoid using this for Media Library content at all costs.