dual boot lubuntu on Chromebook 2, partition header problem

hey all,
I have a Chromebook 2 (Toshiba). After a couple of days of playing around with Chrome OS I decided I wanted at least dual boot.

I followed these instructions: http://chromeos-cr48.blogspot.de/201...em-all_31.html to install lubuntu LTS. All went well, except that at rebooting, the laptop rebooted into Chrome OS.
Researching I found these instructions: https://sites.google.com/site/chrome...developer-mode and added the script that would allow me to boot into ubuntu.
One difficulty was that the partition seem to be named differently than described. /dev/sda couldn't be found. Using rootdev, I found it's /dev/dm-0, so I exchanged that.

Now, obviously I'm still rather green. I'm not sure if the partition I want in the script is the one that rootdev shows.
Either way, what happens is that; if I type "ubuntu" in the developer console, it gives the following error:
"WARNING: Primary GPT header is invalid
WARNING: Secondray GPT header is invalid
ERROR: GptSanityCheck() retued 4: GPT_ERROR_INVALID_SECTOR_SIZE"

I'm guessing something went wrong in the installation since it didn't boot into Ubuntu in the first place. The other possibility is that I gave it the wrong partition to boot Ubuntu from. Or something else entirely?
Would appreciate any help.

Cheers!
Ivo