I've not tried this with my R15, but I have tried it on other radios that say they only support up to 32GB. I've always been able to make it work, even with bigger ones like 128GB. To make it work I've had to make sure they were formatted as FAT32. Note the default file system is exFAT 64GB and larger micro SD cards.
That said, I usually use 32GB micro SD cards as I don't need the extra capacity.
