• Turn off the laptop.
• To invoke Recovery mode, you hold down the ESC and Refresh keys and poke the Power button.
• At the Recovery screen press Ctrl-D (there's no prompt - you have to know to do it).
• Confirm switching to developer mode by pressing enter, and the laptop will reboot and reset the system. This takes about 10-15 minutes.

Note: After enabling developer mode, you will need to press Ctrl-D each time you boot, or wait 30 seconds to continue booting.

• After booting into developer mode, hold Ctrl and Alt and poke the T key. This will open up the crosh shell.
• Type shell to get into a bash shell.
• Type sudo -i to become root. (sudo su doesn't seem to be available and this is functionally the same)
• Then type this to enable USB booting:

crossystem dev_boot_usb=1 dev_boot_signed_only=0

• Reboot the system to allow the change to take effect.

• https://archlinuxarm.org/platforms/armv8/rockchip/samsung-chromebook-plus
• https://wiki.gentoo.org/wiki/Samsung_Chromebook_Plus
• https://wiki.gentoo.org/wiki/Creating_bootable_media_for_depthcharge_based_devices
• https://git.alpinelinux.org/aports/tree/testing/linux-gru/
• https://sites.google.com/a/chromium.org/dev/chromium-os/how-tos-and-troubleshooting/mainline-kernel-on-kevin-chromebook-plus
• https://github.com/hexdump0815/imagebuilder/blob/main/systems/chromebook_gru/readme.md