Problems with the octagon sf8008 multiboot

Unfortunately, I absolutely cannot get the multiboot to work on the Octagon sf8008, despite following the instructions. In fact, I mounted a 16 Gb USB stick like / media / hdd, which was formatted ext4, with the standard directories. I then inserted an unmounted 32 Gb Kingston 100 Mb / s micro SD, which is regularly read by the multiboot and initialized in 5 partitions: sda1, sda2, sda3, sda4 and usb. When I flash a new image it is regularly written on the sda2 and sda4 partitions, but if I restart with the new images they do not start and I keep the original one openatv 6.4. I don't explain the usb partition that shouldn't be there. Can anyone help me? Thanks in advance.