If you may received an error when you mount external hard disk, mount you must specify the filesystem type.

You have to specify  the file system type  "  -t   FILESYSTEMTYPE "  (ext2, ext3, ext4, 4dev, msdos, vfat) argument to the command,

Find out current file system types by command,

​# df -T

Filesystem     Type   1K-blocks      Used  Available Use% Mounted on
/dev/sda2      ext4   226784652   6491384  208773268   4% /
tmpfs          tmpfs     445616       232     445384   1% /dev/shm
/dev/sda1      ext4       99150     57181      36849  61% /boot
/dev/sda5      ext4    14250928  12136220    1390792  90% /home
/dev/sda6      ext4     8063408    532164    7121644   7% /tmp

List out and check your file disk using fdisk command,
# fdisk  -l

Now, create a directory and mount file system /dev/sdb2 to /thelinuxfaq,
# mkdir /thelinuxfaq

# mount -t ext4 /dev/sdb2  /thelinuxfaq

Once executed the above command check the directory,
# cd /thelinuxfaq

Unmount Device

Unmount your device,
# umount /dev/sda2

# umount  /thelinuxfaq

Error :

umount: /thelinuxfaq: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))


This error generated due to you are currently working  in files, come out from that device