External hard disk not showing up on finder and can't be verified in Disk Utility
If you can see your external drive in the disk utility app, there is no reason that it won't show up when you hit 'diskutil list'.

If you still can't see it after 'diskutil list', try unplugging it then 'ls /dev/disk*' then replug it and redo 'ls /dev/disk*' and see if another disk appeared. (It should be disk2s2 or disk2s1) then 'diskutil mount /dev/disk2s2'.

If this doesn't work, your drive is probably corrupted.