The sdcard is already fat32. The problem is with the recovery (twrp 188.8.131.52) provided in the boot.img (previously renamed zImage and putted on the sdcard) on the main page ("new kernel 310mb") that can't mount /external_sd. I get this error messages coming from a fresh install :
E:Unable to find partition size for '/boot' E:Unable to find partition size for '/recovery' E:Unable to recreate and-sec folder. E:Unable to mount storage. E:Unable to mount /external_sd during GUI start up. E:Unable to mount /external_sd/TWRP/.twrps when trying to read settings file.