Congratulations on bumping an 8 month old thread to berate people who have long since forgotten about it.
The age of the thread is irrelevant. If you Google for the answer to whether or not the 3DS works with 256gb cards, this thread comes up. This is how I found it. No doubt others have come here before me looking for the answer to this question as well, and as will more in the future.
So congratulations to you on not knowing how the internet works.
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I own a 256gb for my n3ds and it works great.
Thanks.