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 AND a removable SD card.

From http://source.android.com/devices/tech/storage/index.html:

"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".

What this means is that with KitKat, applications will no longer be able create, modify, or remove files and folders on your external SD card.  As a for-instance, you can no longer use a file manager to copy files from your computer to the SD card over a network.  This ability, which has existed since the beginning of Android, has been taken away.

The only stated reason for this removal of functionality is that, "Restricting writes in this way ensures the system can clean up files when applications are uninstalled."  I do not pretend to understand this logic.  Apps are still allowed to write in arbitrary directories on the primary storage (with the appropriate permission), but are denied the same access to external storage.

Samsung has implemented this feature with their KitKat OTA updates.  Note3 users are now complaining that FX File Explorer can no longer write to their external SD cards.  There are solutions to this problem for users with root access.  Users without root access appear to be screwed.

I'm not quite certain how Google intends for you to place files on your SD card.  Perhaps you have to use proprietary Google apps that contain permissions unavailable to the rest of the developer world.  Perhaps you're supposed to put everything on the cloud and pay carrier data fees to get it there.  Perhaps you're supposed to use some kind of WIRE to attach your WIRELESS device to your computer and have the computer do that work for you.

In my opinion this is a horrible misstep by Google and the Android Open Source Project.  Functionality has been removed without reason, to the severe detriment of users and developers alike.

I apologize for not bringing this to everyone's attention when KitKat 4.4 was released, but it was not mentioned in the Android 4.4 changes document: http://developer.android.com/about/versions/android-4.4.html.  It's only mentioned in the article on source.android.com.  I was only made aware of its existence from user reports as a result of Samsung implementing this change in its KitKat OTA updates.
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