By default, Linux_for_Tegra/tools/kernel_flash/flash_l4t_external.xml only supports a 64GiB SD card and above. If you want to flash a 32GiB SD card, you will need to modify the num_sectors field in Linux_for_Tegra/tools/kernel_flash/flash_l4t_external.xml so that num_sectors * 512 = 32GiB.