I've got a 32gig, and thousands of files, but lately the Nook has been 'losing' the microSD card. One minute it'll be there, then the next it's not. Sometimes rebooting will get it to recognize the card, other times not.
Twice I've had the Nook tell me the SD card is corrupt, but then it can't format it. So I've taken it out and formatted it on my computer, reinstalled it and then formatted it through the Nook.
Then reload it, only to have it happen again a few days later.
I'm trying to figure out what triggers the problem, but don't have a clue so far. This is a high speed card, and loads quickly. So lag time shouldn't be the problem but I think it happens first when I connect to my computer and it starts loading and unloading the Nooks main drive and then the SD card. After that, the Nook can't see it either.