I use this code for uEnv.txt, my SD is named ‘BBB_Ext’:
mmcdev=1 bootpart=1:2 mmcroot=/dev/BBB_Ext ro optargs=quiet
I use this line in my fstab file:
/dev/BBB_Ext /media/card auto auto,rw,async,user,nofail 0 0
It boots with 3 LEDs and no connection to the computer. Without the SD card, it is detected and I can SSH properly.
Help greatly appreciated!