Z69 doesn't boot

I own a z69 box (Amlogic S905x, 3GB ram). I can boot kaszaq’s old libreelec builds, armbian (balbes) and the experimental libreelec build by balbes with roughly the same steps:

  1. pv imgage.gz | gunzip | dd bs=128k of=/dev/sdx
  2. mount /dev/sdx1 /mnt/test
  3. cp /mnt/test/device_tree_folder/gxl_p212_3g.dtb /mnt/test/dtb.img
  4. umount /mnt/test
  5. :arrow_right: insert sd card into tv box and reboot

But somehow it doesn’t work for CoreElec. The box will always boot to the internal emmc. Does CoreElec use a different boot method? How to debug it? It’s not problem of the sd card or the tv box. They work for all images except CoreElec.

Have you tride this method

[8.0.2e] LibreELEC 8.0 for S905/S905X

I’ve got it. The bootloader (!?) seem to be incompatible. I have to repeat the reboot update step.
And once I’ve done it, a sd card with e.g. armbian won’t boot. I would have to install the armbian bootloader (!?) again.
This wasn’t necessary before with LibreElec and armbian. Is there a way to avoid this?