Hi, currently I'm using SanDisk 64GB microSDXC card and it doesn't seem to work. System > Stores > Disk shows the microSD as invalid and formatting takes a very long time. After being fed up of waiting, I rebooted the router and the microSD card doesn't seem to be recognized anymore. Bummer..
After reading tomaskir's comment, I shut down my router, rebooted it and reformat the card. Now I can confirm that SanDisk 64GB microSDXC is indeed compatible with RB1100AHX2!