Why does the device tell me "not enough storage"?

So my device is supposed to have 16 GB but apparently it is devided in 2? 788mb and 11.something GB . I literally only have 4 apps downloaded and I can't download anything else because it tells me not enough storage but I check and the storage bar that says 788mb isn't full and the second storage bar that has 11. GB is practically empty! What's going on?!? I can't find any way to save things to the second storage bar or move the files to it. Not to mention I have an external memory of 12GB in my SD card on which my apps are stored so WTF?