First use diskutil list to list all avaiable disks and identifiy the external hard drive to mount. For example, running diskutil list on my Mac gives me the following output and /dev/disk2s1 is the partition (NTFS) to mount.

$ diskutil list
/dev/disk0 (internal):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme …