The max size is 32GB. This is the limit for all types of SD cards on Arduino. Arduino only supports SD (2GB and less), and SDHC (more than 2GB and up to 32 GB).Cards larger than 32GB are SDXC and are not supported by arduino libraries.SDXC cards are pre-formatted with Microsoft's proprietary and patented exFAT file system. Microsoft does not publish the specifications of exFAT and its use requires a non-free license. Therefore I have not attempted to support SDXC cards in SdFat. SdFat is the base library for the Arduino SD.h library.
I am surprised to hear about the Microsoft proprietary format.
Bill, have you ever had any success with formatting SDXC cards with FAT32? Best regards, Dani