http://beranger.org/2014/01/30/how-kitkat-4-4-killed-android-and-google-keeps-screwing-things/
KitKat will make your SD Card completely useless: per the Android API specification, apps can no longer write files to your SD card. And Samsung is following it.
This only applies to dual-storage devices, i.e., devices with a user-writable internal flash storage ANDa removable SD card.
From External Storage Technical Information:
“The WRITE_EXTERNAL_STORAGE permission must only grant write access to the primary external storage on a device. Apps must not be allowed to write to secondary external storage devices, except in their package-specific directories as allowed by synthesized permissions.”
If your device has user-accessible internal flash storage, your SD Card is a “secondary external storage device”.
ps. 위에 지정된 디렉토리 이외에 앱들이 쓸 수 없다는 것은 캐시 등의 앱 데이터에 관한 것으로 알고 있습니다.