I purchased an old EOL Asus C740 Chromebook. EOL means that it will no longer receive updates. That will eventually be an issue when the chrome browser version will no longer be supported. So I decided to try and update it with Flex OS.
https://www.wikihow.com/Enable-USB-Booting-on-Chromebook
The link above was the starting point in my journey. It needed a few tweaks.
This process will wipe all data the data on the internal hard drive. Backup data if you need to.
It is recommended to make a bootable Flex OS boot usb. You will need it later. https://support.google.com/chromeosflex/answer/11541904?hl=en You can boot other bootable USB sticks, but not all will work. Venroy, Ubuntu, and Zorin are three that I have managed to boot from. But a Chrome OS (Flex OS) is likely to work best.
This first step you might not need to do. But I did. Without doing this, the Control and L key combination mentioned below did not work. It was blank screen. So I had to hold esc and f3 (the third key after the esc key) then pressing power. This brings you to the Recovery Mode. Then with an original Acer C740 Recovery USB, that I made before, I reinstalled the original ChromeOS again.
So after reinstalling the ChromeOS hold esc and f3 (the third key after the esc key) and pressing power again. Note: The only usb image that will work on this screen is the original Chrome OS image for this model. I would recommend you make a Recovery USB for your model. Someday it may no longer be available.
Now you need to press Control and D to activate the developer mode.
This will allow us to first turn off the OS Verification.
After we have turned turn off the OS Verification, hit Control and D again. It should start “transitioning to Developer Mode”. It will take about 20 minutes.
On the next reboot, at the white ChromeOS splash screen, press Control, Alt, and F2 (the 2dnd key after the esc. This changes the ChromeOS terminal from the GUI Terminal to a text terminal. You could go back using, Control, Alt and F1, but don’t do that now.
In the terminal login with username chronos and no password.
Then type: sudo su -
Then: crossystem dev_boot_usb=1 dev_boot_altfw=0
Your Chromebook may vary.
Insert your bootable FLEX OS USB flash drive you will have made previously.
Restart your Chromebook. Your Chromebook will reboot once again to the warning screen.
Press Control and L. It should ask you to hit esc. Do so. Then chose the drive to boot from. Later, you will need to do this step everytime you reboot your PC. Control and D will not boot your PC off the internal drive as we will have reinstalled Flex OS on it.
What I did at this time was install Flex OS over the original Acer Chrome OS. You do not need to do the same. You can leave the original Acer Chrome OS on the hard drive and just boot off usb images when you need to.
Note that https://docs.mrchromebox.tech/ has other options. They involve taking the back off the back of your Chromebook and removing a screw so you can overwrite the firmware. There is a lot of information on that site, and it can be quite daunting. THIS IS A ONE WAY PROCEEDURE if your chromebook is EOL. The Acer C740 is. So only do this if you do not want to revert your chromebook back to the original configuration. I want to be able to revert back to the original state. So that is why I stopped here. Mr. Chromebox has taken the stance to not support EOL chromebooks with all the feature that he provides for newer chromebooks. That is his choice. But I find it an odd decision.