Background
In Android 4.4 (KitKat), Google restricted third-party (non-preinstalled) applications from writing to external/removable storage. This decision crippled many applications, making some useless. Pressure on Google led to a compromise solution integrated into Android 5. Applications can regain write permission to external storage using a new API with the SAF (Storage Access Framework). However, generally, external SD cards are always much less reliable (especially certain device/SD card combinations) than internal memory.
First Start

- A directory picker will appear. You need to select the directory (or volume) you want to work with.
- Tap the More button (top-right corner) and choose Show SD card.
- Tap the Menu button (top-left corner) and choose the storage you want to use.
- Tap SELECT (bottom).

Limitations
There are some limitations in SAF mode you should be aware of:

- The SAF API provides much lower performance than standard direct file access mode (especially with directories containing many files/subdirectories).
- Home/Custom directories cannot be set in SAF; however, the last-picked SAF URI is automatically saved: Home Button → Go to Last Picked SAF Dir.
- The wipe feature does not work. If the “Wipe after Encryption” option is checked in settings, files will only be deleted.
- The “Open File” function will only work if the destination application can decode the SAF URI.