By default, the Chromium OS rootfs is read-only. If you boot the system in developer mode, you will be able to disable rootfs verification and modify existing files or write new files into the file system. Before you do this, note that your file system will no longer be verifiable (won’t checksum properly) and you’ll end up needing to restore a recovery image in order to get back to normal mode. So this might be a bit dangerous if you’re not using the device for something like regression analysis (why I needed to do this).
To make the file system writeable, first fire up a command prompt via crosh, by using Control-Alt-T and then running the shell command at the shell prompt:
shell
Then run the following shell script to remote rootfs verification and make the file system writeable:
sudo /usr/share/vboot/bin/make_dev_ssd.sh --remove_rootfs_verification
Then you can reboot and do whatever it is you need to do. For example, install a vine server and run automation scripts to do regression testing. Enjoy.