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

or
 
# 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))

Solution:

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