I've had my SG3 for about 2 months and all i do is read and write to my external sd card. All my music is stored on it (i listen to music constantly), I have movies and tv shows on it (which I swap out frequently) and anything the SG3 camera does (video and picture) goes right to the sd card. Never a hiccup or anything.
I think it would really help this convo if people start adding in if they are rooted, if they are using a custom rom, or forcing apps to the sd card.
I'm unrooted and stock everything. no customization apps.