ohh 64gb cards(or larger) need a specific pc program to format them before they will work properly with 3ds cfw
sorry i dont have a link
btw a lot of 64gb or larger sd cards from what i read are fakes (a fake card will claim to be 64(or larger)but in fact be 32gb in size) to avoid getting a fake card buy it at a store not ebay or amazon
My card is a Transcend 64gb card that I got direct from Amazon.
Specifically, Windows Vista and above do not allow volumes larger than 32GB to be formatted as FAT32 (only exFAT, which the 3DS does not support). The easiest way to do it is to use a program like fat32format to format the card. Alternatively you could use a MAC or Linux system to format it.
A gui version of fat32format can be downloaded from the developer's website here: http://www.ridgecrop.demon.co.uk/index.htm?guiformat.htm
I've been using the HPUsbformat tool to format the sd card. I'll give fat32format a try.