To install or boot a custom Linux environment right now, you'll need to enable developer mode, open the Crosh shell, and type the appropriate commands to download and run the third-party scripts that will install Linux for you. All enabling developer mode does is give you access to the system files. "Developer mode" was literally a physical switch on the original Chromebooks, and flipping that switch just let you modify the system files.
No more. Enable "Developer mode" on current development versions of Chrome OS and you'll get a few new options, courtesy of some recent additions.
USB and SSH
The "Boot from USB" option will allow you to activate booting from USB devices, and you'll then be able to insert a USB drive with a Linux system on it and boot like you would on a typical computer. This would allow you to boot a full Linux environment from a USB drive without modifying your Chrome OS system.
Other options allow you to enable secure shell (SSH) access and set a custom password you can use to access it. You can then use an SSH client to access the Chomebook's command-line "shell" from another computer. Theoretically, it would be possible for a Linux-installation tool to run as a desktop program, ask for your Chromebook's IP address and SSH password, and then SSH into your Chromebook and automatically install and configure the Linux system.
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You could then access that powerful Linux system in a Chrome OS desktop window thanks to another awesome tool from Crouton developer and Google employee David Schneider.
These new debugging tools are useful for developers, sure. But they're also useful for us enthusiasts looking to run Linux on our Chromebooks. These changes were explained by Chrome evangelist Francois Beaufort on Google+.
How to enable the debugging features
These options were just added to the development version of Chrome OS, so they should show up in the stable version 41 of Chrome OS. For now, you'll have to switch your Chromebook to the "dev channel" and update to the latest, unstable code to get these features.
After switching to the dev channel and updating your Chromebook's operating system, you'll need to perform a "powerwash" and completely wipe your Chromebook. This is a security measure, ensuring all personal data is off the Chromebook before its software is cracked open. It prevents attackers from abusing developer mode to gain access to all the sensitive data stored on a Chromebook.
Enable developer mode and continue normally. After pressing Ctrl+D to bypass the developer mode warning message, you'll see a new "Enable debugging features" link that allows you to enable these useful features. It's right at the bottom of the normal first-time setup wizard. Check out the Debugging Features page on the Chromium project's official wiki for all the details.
That's it for now! These options--especially booting from USB devices--should pave the way for a whole new generation of easier-to-use full Linux systems for Chromebooks. If Chromebooks are the consumer Linux desktop we were all waiting for, it's great to see Google making it easier to turn a cheap consumer Chromebook into a power-user's Linux desktop.