In v66 and higher, CloudReady is adding a security feature found in Chromebooks called rootfs verification.
Rootfs verification's ensures that the OS is not tampered with between reboots by preventing edits to the root drive and halting the boot process if an unexpected change is detected.
If you want to edit the root file system on purpose, it is necessary to disable this feature first.
v66 and higher
If you are using the Home Edition and you want to purposefully make edits to your root file system, you'll need to take an extra step in v66 or higher to disable rootfs verification prior to starting any changes.
Note that disabling rootfs verification will expose you to the risk of your disk being tampered with between reboots.
Steps to disable rootfs verification:
1) Log in
2) Press ctl+alt+t to open a terminal
3) Type in "shell" and hit enter
4) Confirm that the yellow "crosh" prompt is now a green "chronos@localhost" prompt
5) Disable rootfs verification:
6) Reboot your device:
7) Now you can log in and proceed with rootfs edits as you normally would. You will still need to remount root as read/write:
sudo mount -o rw,remount /