I've been using a Sandisk Ultra 64GB Class 10 Micro SD Card in my Fuze+ since September 2013 and it works fine. Before that I had used a Transcend 32GB microSD. I know there can always be glitches with the player hardware and even the drivers but as far as I can tell if you use a good quality card with sane partition table it will work very well in Rockbox. About the worst thing you can do is use a card of dubious origin. The next worst thing is to partition or format it in ways that differ from the recommendations of the SD Association. Be kind to yourself and just use their format tool. Also avoid rap music.