Well to clarify this point.
there is no built in external memory. there is only internal sdcard and external sdcard.
Your internal storage is sdcard. sdcard and sdcard0 are the same thing, they are linked together if you delete seomthing from sdcard it will disappear from sdcard0
extsdcard is your external sdcard.
Google has done this by design for a couple reasons that frankly are above my head. I know one of the reasons is because they dont want external storage for whatever reason, this can be seen in their nexus devices. But whether they like them or not most apps from the market are set up to send their files to sdcard, so google had to keep the storage directory called sdcard.
Now Samsung gets Jelly bean and wants to add external sdcard option (hooray for that), but they can not call their sdcard simply sdcard because this directory has already been designed into the system to deeply and they cant just change it. So they call their sdcard extsdcard.
This renaming of directories is what is causing the "mess". Apps look for sdcard which intern fills your internal memory. The only way to fix this is to find settings on each individual app (if available) and direct it towards your external sdcard. for example your music players you need to go to settings and tell it what folders to look in . Or your camera you need to select external memory....etc
I dont know if this is an answer to your question , but maybe more of a reasoning to the seeming mess of storage.
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