Cannot mount HDD topic

Dear All,

Variant of this problem may be common but I couldn't solve it.

I have a 1 TB Silicon Power External hdd. It is formatted with gparted, using ext4 and gpt.

Everything was working fine till today. I unmounted it with

sudo umount /media/directory

Since then, I cannot mount it by any means.

Here I post the outputs of commands I tried.

Code:

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sudo fdisk -l

Disk /dev/sda: 500.1 GB, 500107862016 bytes
255 heads, 63 sectors/track, 60801 cylinders, total 976773168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk identifier: 0x0004d6da

  Device Boot      Start        End      Blocks  Id  System
/dev/sda1  *        2048    97867775    48932864  83  Linux
/dev/sda2        97869822  976771071  439450625    5  Extended
Partition 2 does not start on physical sector boundary.
/dev/sda5        97869824  898648063  400389120  83  Linux
/dev/sda6      898650112  976771071    39060480  82  Linux swap / Solaris

Disk /dev/sdb: 1000.2 GB, 1000204886016 bytes
255 heads, 63 sectors/track, 121601 cylinders, total 1953525168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk identifier: 0x000bf9e3

  Device Boot      Start        End      Blocks  Id  System
/dev/sdb1              63  1953520064  976760001  83  Linux
Partition 1 does not start on physical sector boundary.

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Code:

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sudo mount -t ext4 /dev/sdb /media/peptid/
mount: wrong fs type, bad option, bad superblock on /dev/sdb,
      missing codepage or helper program, or other error
      In some cases useful info is found in syslog - try
      dmesg | tail  or so

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HTML Code:

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/usr/bin/udisks --mount /dev/sdb
Mount failed: Error mounting: mount exited with exit code 1: helper failed with:
mount: wrong fs type, bad option, bad superblock on /dev/sdb,
      missing codepage or helper program, or other error

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I opened gparted, and it says 'filesystem is unknown'

Code:

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/usr/bin/udisks --mount /dev/sdb --mount-fstype ext4
Mount failed: Not a mountable file system

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I have very important data in it, so I have to save.

Any help would be greatly appreciated.