Black does not boot from emmc anymore after inserting/configuring micro sd card

My Black doesn’t boot Debian from emmc anymore …

What i was doing:

    • inserted a micro sd card
    • created a etx3 filesystem on it
    • edited fstab (mount -a works as expected, ext3 partition was mounted)
    • sudo reboot (not pressing any button during restart)

Thats all. Before doing this, it boots from emmc. I wanted to use the micro sc card as extra storage, not as boot device. I can’t get the point what happened here. Atm, the Black only shows heartbeat LED without the micro sd card in slot.

I could now …

  1. buy a micro hdmi adapter and take a look at the output
  2. using a normal image on the micro sd card to boot from and correct the boot settings. Where i have to look?
  3. … some magic you may know

Any help is greatly appreciated.