Beginning with KitKat, Google has changed the way third party applications can interact with external storage, such as an SD card. This may affect Apps that were installed on SHIELD Portable prior to the KitKat upgrade.
1) SHIELD Portable’s “Move Apps to SD card” feature (Settings -> Apps ->[Select App]-> Move to SD Card) works the same as it does on Jelly Bean. Apps that you have moved to an SD card on SHIELD Portable using this feature should still be on SD Card after you update to Kitkat.
2) Third party Apps (such as ES File Explorer) no longer have general access to write to the SD Card in KitKat. This was a change in behavior made by Google for security purposes and is not something we are allowed to support or work around starting with Kitkat.
3) Apps CAN write to app-specific folders on the SD Card using a new API available in Kitkat. For instance, App-XYZ can write to its own folder on SD Card, but cannot write to App-ABC’s folder. Note that your Apps may need to be updated to support this SD Card write capability.
There is an article on Android Central that provides a plain English overview of the change – worth taking a quick read before you upgrade.
You can also discuss this change on our SHIELD Portable forums.