The SD card needs to be formatted to the primary FAT32 MBR partition type.
While the included SD card is 16 GB, cards up to 128 GB were successfully tested too. Some of the Tracker users are reporting the use of cards up to 512 GB. We recommend UHS-I cards, particularly ones that are rated A1 or A2. The built-in disk-utility under Mac OS will allow you to select FAT32 for formatting.