Can you please paste the output from
This will show you which block devices are recognised by the system.
A couple of points. I usually mount in /etc/fstab by UUID - it's more precise as the system may arbitrarily assign different device names on startup.
Secondly, do you really want to to mount the filesystem directly into /mnt? Don't you want to mount it into a subdirectory? Usually I create specific folders under /mnt (well, /media on my machine) and use those as fixed mounting points for my filesystems.
A sample line from my fstab:
UUID=86ff5f7a-02a3-e101-00bc-529202a3d101 /media/sharelnx ext4 rw,auto,user 0 2