You can't mount a harddrive, you mount a file system. My guess is you haven't created a file system on the external drive yet. If you haven't created a file system and formated the drive, do that first. You can use any number of utilities to do this ie. fdisk, cfisk, parted magic. After the file system is created and formated then create a mount point to mount to.
as root or sudo
mount /dev/sdb1 /mnt/name
where name is the name you want to call the mount point.
If you want the drive to mount auto at boot you'll need to edit you /etc/fstab