It absolutely sucks that Android has prevented most apps from writing to the external SD card, as can be read here: AnandTech | Examining MicroSD changes in Android 4.4. Apps can write to their sandboxed areas, so Google Music sync works fine and stores GBs of music on the external SD card at /storage/extSdCard/Android/data/com.google.android.music. However, I sync my photos to my phone via FolderSync and with many GBs of photos can only do this to the external 64GB SD card I installed on my Samsung S5. Samsung's recent update of Android 4.4 prevented this working and has forced me to enable root on the phone and then follow the instructions at Unlock external SD Card writing for all apps in Android 4.4 KitKat.