Unable to mount an EBS volume as the root volume in Amazon EC2

I have an Amazon EC2 instance that seems to be messed up.

It has 2 disks - 8GB and 80GB. 8 GB seems to be the bootable disk but the entire filesystem/data is on the 80GB hard disk. If I try to boot the server with the 80 GB disk to /xvda, the server does not boot up. What I am looking for is to boot up the server with the 80 GB disk mounted in the root directory /. I am able to mount it in a folder but not in the root folder.

Output of lsblk

[root@ip-10-0-0-29 ec2-user]# lsblk

NAME    MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
xvda    202:0    0   8G  0 disk
└─xvda1 202:1    0   8G  0 part /
xvdb    202:16   0   4G  0 disk
xvdf    202:80   0  80G  0 disk 

[root@ip-10-0-0-29 ec2-user]# df -h

Filesystem      Size  Used Avail Use% Mounted on
/dev/xvda1      7.8G  895M  6.8G  12% /
devtmpfs        1.9G   64K  1.9G   1% /dev
tmpfs           1.9G     0  1.9G   0% /dev/shm

I tried to use a Label and Labelled the 80GB disk as mainboot and added the following in the /etc/fstab file to mount the disk on startup, but that doesn't do anything. The 8GB disk is the one that gets mounted on startup to the root folder.

Contents of the /etc/fstab file

LABEL=mainboot    /           ext4    defaults,noatime  1   1
tmpfs       /dev/shm    tmpfs   defaults        0   0
devpts      /dev/pts    devpts  gid=5,mode=620  0   0
sysfs       /sys        sysfs   defaults        0   0
proc        /proc       proc    defaults        0   0

I would like to know the easiest way to boot with the 80 GB disk. This used to work earlier as the entire linux filesystem is present in the disk, but I'm not sure why I am unable to boot using it.

Also unsure if its a AWS related issue or Linux related. The issue cropped up when I was trying to upgrade the AWS instance from medium to large.