You are probably running a v0.3.3 image on the eMMC and you just flashed a v0.3.4 image on the SD card. In order to boot from the SD card for the first time, follow the instructions here:
you have to hold press the "USER" button on the BeagleBone Black (or equivalently short P8_43 to P8_01 if it is more convenient). Then the board will be forced to boot from the SD card.
At this point you have two options:
- if you want to keep the eMMC as it is and only use the SDcard from time to time, run /opt/Bela/bela_bootloader.sh
once. After reboot, it should boot from the SD card with no problems, and you will still be able to boot from the eMMC if you so wish.
- if you want to copy the SDcard to the eMMC, run: /opt/Bela/bela_flash_emmc.sh
. This will take about 10 minutes and display Done!
when done. Occasionally it fails, at which point you should just run it again and it should work.