An app attempting to write to the SD on an Android 5.0 device will prompt you to grant access.
Then select the card. Don’t navigate down into any subfolder. You MUST select the top-level root folder of the SD card.
After tapping the “Select SD card” button you should be back in the app. The app double checks if it now indeed can write to the SD card. You should see a message about the check result in the app.
If you replace the SD card in your device, you need to go through this procedure again. The write permission is granted individually for each app and each physical SD card.