Default SD Card problem

So I got a new SD Card, a 16 GB one, I bought this one to replace my old, junky, 4 GB card.
Well so of course, I made a backup of the old one and then I just pasted the files on the 16 GB ones, everything, music, images, etc.
Well then I tried to open my pictures, but it said I had none and I checked the apps and some had the SD Sign (meaning they're on the sd, but it's weird since my model has something like a internal storage, a storage just named "SD Card" (in this one every app goes in there), and a final one named External SD Card (which is the one im talking about here)), then I tried checking if it had any data into it but it still said I had 16 GB free of 16 GB even though there was like 2.3 GBs used. Well, then I checked the old SD's storage, it had 2.80 free or something like that (the same space it had before I even deleted the files), then I just copied the files to the old one again and everythings working again fine. But I really need the 16 GB one to add some more stuff. Does anyone know what does this mean?