Use fork of SdFat that supports gpt partitions so users do not have to format their cards with mbr. Thanks @PetteriAimonen for the patch.

platformio.ini

@@ -6,7 +6,7 @@ default_envs = STM32F1
 [env]
 framework = arduino
 lib_deps =
-    greiman/SdFat @ 2.2.0
+    SdFat=https://github.com/BlueSCSI/SdFat#2.2.0-gpt
 upload_protocol = stlink
 ; Different gcc versions produce much different binaries in terms of speed.
 platform_packages = platformio/toolchain-gccarmnoneeabi@1.90301.200702