General :: How To Add A New Entry To Fstab

I mount a device mount /dev/xvdf /mnt/mongo and verify the mount using df-h:

Filesystem Size Used Avail Use% Mounted on
/dev/xvda1 7.9G 955M 6.9G 12% /
tmpfs 299M 44K 299M 1% /dev/shm
/dev/xvdf 20G 589M 19G 4% /mnt/mongo

But now I'm trying to figure out how to make it auto mount on boot. I understand I need to add a new entry to /etc/fstab, so I perform: $ sed -i '$ a/dev/xvdf /mnt/mongo xfs defaults 1 1' /etc/fstab But, after reboot, it seems that the auto mount didn't work. The device didn't appear in the df -h list.