At the moment both UHS-I SD cards and non UHS Cards are supported. UHS Cards operate at higher speeds and use 1.8V instead of 3.3V. (1.8V now works, we found LDO5 in the PMIC which handles SD signaling voltage)
The left SanDisk Card is UHS-I (SDXC-I) and has SpeedClass 3 (3 in U) and VideoClass30 (V30) See https://www.sdcard.org/developers/overview/bus_speed/index.html The right (Intenso) Card is an old Class10 (10 in C) SD card which works.
Use GParted or similar tool to create 2 partitions on your SD Card. First partition FAT16/32 - 255MiB Second partition EXT4