I have what is presumably a rather old BBB and I can only get it to boot from the SD card image by holding the SD (?) button down when powering up. I need it to boot from the SD card by default but I don’t want to get rid of the EMMC card contents yet.
I seem to remember there’s a change somewhere in the boot system to boot from SD card if there is one.
Can anyone point me at what I need to change please.
This seems to have resolved itself, having booted Debian 11.7 from the SD card it now reboots to Debian 11 without any external intervention. Presumably uEnv.txt has been changed as required. 
I spoke too soon! When I reboot the Debian 11 system it reboots to Debian 11 but when I power down and power up again I get the old system from the EMMC.
So, my question still stands, I need to know how to make the system boot from SD card when one is present and only boot from the EMMC if there is no SD card.
Normally the easiest way to make the SD card boot first is to wipe the first few Megs of the eMMC.
An alternative method is to modify the uboot scripts when booting from eMMC and get it to load stuff from the SD card instead of the eMMC.
I have done this before on another dev board.
Not done it on a BBB though but it is possible for sure.
If you stop uboot by pressing space you can view the various uboot scripts that are run. Test it and if it works do it again but save the environment so it sticks across power cycles
2 options…
A new enough bootloader in eMMC that understands /boot/uEnv.txt
Or look at the schematic, the “boot” button is wired to the header, just ground that pin…
Option 1 should work for any image on the eMMC that’s not older then 2020 at this point…
Show us your bootlog with only the eMMC present, we can help pinpoint how old you are…
Regards,
Option 1 should work for any image on the eMMC that’s not older then 2020 at this point…
I suspect my EMMC has an image older than that, it’s a Debian 9 system still with a 3.8.x kernel.
I think I can probably muddle through as I am, I am actually on the boat where the BBB is at the moment and I will be for a couple more days, so I can push the button when needed. Then, before I leave and only have remote access, I will copy the SD image across to the EMMC. I’ve already got the ssh tunnel access working on the Debian 11.7 image so I can access it remotely. It’s hardly a major disaster if it all goes pear shaped, I’ll be back again in a few weeks time.