Mounting persistent drives makes Google find the instance inaccessible

I followed google docs from scratch, ( https://cloud.google.com/compute/docs/disks/add-persistent-disk )

to set up a permanent drive in my compute instance, everything goes fine until I add it to / etc / fstab as the doc suggests.

echo UUID=`sudo blkid -s UUID -o value /dev/disk/by-id/google-[DISK_NAME]` /mnt/disks/[MNT_DIR] ext4 discard,defaults,[NOFAIL] 0 2 | sudo tee -a /etc/fstab

If after rebooting I reboot, my instance will become un-ssh-able. (although it appears as running in the console)

I repeated the same steps in the centos 7 image, 5-6 times from scratch from a new instance, and each time the echo in fstab makes my instance inaccessible. terrible terrible dock. but i still want to know how can i fix this?