Disk root error on start instance on “Amazon Web Services (AWS)” EC2

Not sure if this will work but instead of what you did above.

Attach the old EBS drive instance to /dev/sdb2 and don't detach the current /dev/sdb1 as this is running an instance that you can still SSH into.

Inside the new running instance you should be able to run

sudo mount /dev/sdb2 /mnt
chmod /home/ubuntu/.ssh 700

# assumes the home directory is the default ubuntu one 
# for ubuntu/debian instances.

Followed by:

sudo umount /mnt

Now detach the drive in EC2 console and reattach it to your old instance and start it back up. Hopefully now you'll be able to ssh back in.

I created a new instance by going to Images > AMIs > Private Images > Selecting the image the instance was started from > Launch. I launched in exactly the same availability zone, not just US or region, but the 2a, 2b, 2c must match as well. I stopped the new instance. I disconnected the EBS volume from the old instance. I re-attached the EBS volume to the new instance at /dev/sdf. I started the new instance. The EBS volume shows up inside the instance as /dev/xvdj so I mounted it with mkdir /xvdj; mount /dev/xvdj /xvdj. I edited /xvdj/boot/grub/grub.conf and changed default=0 to default=1. I saved the file, stopped the new instance, re-attached the EBS volume to the old instance and it started. I ran yum update in the old instance and double-checked /boot/grub/grub.conf and double-checked that it would reboot.

I also found this regarding updates to CentOS kernel: grub.conf missing initrd path after kernel update

I noticed after I ran yum update I now had 2 entries in grub.conf without initrd. Running # yum reinstall kernel.x86_64 works to fix that.