Booting a converted RPi3 image from SDD

General Discussions
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Thinking a bit more about this, I now realize that it is probably not as easy as initially suggested.

On Raspberry Pi 3 we use U-Boot as boot loader which is required to integrate Mender to be able to switch between partitions.

The integration that is done in U-Boot is pretty much hard-coded to to use SD as boot medium. You can find the U-Boot integration here.

To support booting from SSD (from USB) you would need adjust U-Boot to:

  • Store the environment on SSD (USB storage) instead of MMC. I am not sure if this is even possible, as SSD is a USB storage device in U-Boot which is a “hot plugged”
  • Adjust the mender_config_defines.h to match booting from USB instead of SD
  • Adjust the boot command script to load files from USB instead of SD
    • This is the boot.scr which is generated here

On a stock Raspbian SSD boot probably works out-of-the-box but this is because they do not use U-Boot.