BBB rev C boot from SD problem

Hi to all, I have a problem with rev C of BBB, I have an SD correctly prepared to boot that works as expected on rev A to B boards but that doesn’t work on rev C boards, when the bone boot it will always boot from flash and if I push the boot button it doesn’t start at all, the SD is identified after the boot from flash but fdisk -l tells me that the device doesn’t contain a valid partition table. Anyone knows how to solve this issue ?