There are different options for booting from an external device (useful e.g. to reflash the eMMC)

• Boot the same image from an USB-stick
• Boot image from USB-stick via serial console
• Boot from USB-stick with unbootable eMMC

Troubleshooting

• Only the two USB-2.0 ports can be used for booting:
  • One is the single port on the upper right side
  • The other is the port directly near the power plug (at the bottom left)
  • The port near the powerplug works very unreliable for me, but that may not be the case for you

This is the easiest way, but does not always work.

If the image that you want to boot is the same as the one on the device, then you can simply insert the USB-stick and reboot.
Afterwards, log in via SSH and check, if you have really booted from USB-stick:

mount|grep sda

If you have successfully booted from the USB-stick, the output will read

/dev/sda2 on / type ext4 (rw,noatime,errors=remount-ro,commit=1)

If the output reads

/dev/mmcblk1p2 on / type ext4 (rw,noatime,errors=remount-ro,commit=1)''

then you didn't have luck and need to try other ways.

Background info:

• This is not an intended feature, but rather a bug/exploit
• If the images are identical, the partitions will have identical UUIDs.
• During boot, the initrd looks for this UUID to identify the partition to boot the rest of the system from.
• When there are two identical UUIDs, the first one found will be used (or the last one? doesn't matter)
• With some luck, the system partition from the USB-stick was booted (/boot will still be mounted from /dev/mmcblk1p1)

This is the way shown in https://github.com/TomasOlsson/BrickedSv07-Sv07Plus