Same problem here (sd card unexpectedly removed) on Note II since few hours ago, without even using phone at the time. My card is 32GB SanDisk... The card still works, but gives me problems each time phone would go to "sleep". And every now and then it says its unreadable, so I have to remount it.
I think there might also be problem in Sandisk, since 99% of failed cards are sandisk made 32GB+.
I'll use warranty if it fails, but will also buy 32GB Samsung card, as I havent read one single problem regarding Samsung cards with Note II/SIII. It says somewhere that u should use Samsung microsd, so while I wont claim Samsung did something on purpose, but there is certainly a pattern. Maybe there is some minor difference in voltage or who knows...
Edited to add this:
I am now reformatting it to exFat, as by default it was in Fat32, but I doubt that will solve the problem. There goes 30EUR...