this is what FDC say:

based on the error on the screen and what you said you did, it sounds like you
forgot to format the drive. after partitioning it you added the line in fstab
but since the newly created partition was not yet recognized by the OS, you
couldn't format it. then the server was rebooted and when trying to mount the
newly added /backup, it failed because it does not contain a valid filesystem.
i removed the /backup line from fstab, make sure to format the drive and mount
it manually before adding it to fstab again.