Expandable storage is a benefit of many Android phones, but research by Android Police has revealed that in Android 4.4 (KitKat), Google is restricting the ability of apps to freely write to micro SD cards .
Photo by planet coline
The changes (which you can read about in-depth at the source link below) now mean that apps by default get their own private folders on your phone's built-in storage, but must request permission to write to the SD card. Even with that permission, the app will only be able to read public folders on the SD card. It can't write to anything on the SD card outside of its designated sandbox.
The implications for the change aren't fully clear yet, but for certain devices with an SD card slot and Android 4.4 (including the popular Galaxy S4), some apps may simply be unable to do anything but read data stored on the SD card. While root users may eventually be able to get around this, it's also unclear what would be required. It may be possible for a root app to request the access required, or it may require changes to the ROM itself. For right now, though, if you need to dump some files on to your phone, your phone's internal storage may remain the only viable option.