I have Ubuntu Linux 12.04 version the latest right now.I want to mount an external HDD NTFS 1TB.I have followed many guides but still no success.The error I'm getting is this:

Failed to read last sector (1953523119): Invalid argument
HINTS: Either the volume is a RAID/LDM but it wasn't setup yet,
   or it was not setup correctly (e.g. by not using mdadm --build ...),
   or a wrong device is tried to be mounted,
   or the partition table is corrupt (partition is smaller than NTFS),
   or the NTFS boot sector is corrupt (NTFS size is not valid).
Failed to mount '/dev/sdb1': Invalid argument
The device '/dev/sdb1' doesn't seem to have a valid NTFS.
Maybe the wrong device is used? Or the whole disk instead of a
partition (e.g. /dev/sda, not /dev/sda1)? Or the other way around?
Using Storage Device MAnager i get this error:Error mounting: mount exited with exit code 1: helper failed with:
mount: only root can mount /dev/sdb1 on /media/Skliros_Diskos {external disk name} 

When I use sudo fdisk -l, this is the output:

Disk /dev/sda: 320.1 GB, 320072933376 bytes
255 heads, 63 sectors/track, 38913 cylinders, total 625142448 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000e0bc6

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *        2048   618854399   309426176   83  Linux
/dev/sda2       618856446   625141759     3142657    5  Extended
/dev/sda5       618856448   625141759     3142656   82  Linux swap / Solaris

Disk /dev/sdb: 1000.2 GB, 1000202043392 bytes
255 heads, 63 sectors/track, 121600 cylinders, total 1953519616 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x0002093a

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1            2048  1953525167   976761560    7  HPFS/NTFS/exFAT