It's NTFS. It's USB2. I'm using Ubuntu 13.04. It works perfectly fine on Windows (which excludes cable and hardware problems). I have two Ubuntu computers and it's not detected on either. It's about 500 GB.

Edits:

Following the first link, I input sudo lsusb in a terminal; before and after connecting the HDD. The difference was Bus 001 Device 012: ID 14cd:6116 Super Top M6116 SATA Bridge. There it is! ("sata bridge" used to appear in a windows notification when I plugged in the HDD in!). ...This means that Ubuntu detects it but is it not mounting it?

I tried this:

sudo mount /dev/sdb1 /mnt

But gives this:

mount: special device /dev/sdb1 does not exist

I also tried:

sudo mount /dev/sdc1 /mnt

but it stays with no output forever. I left it in background for about 30 min.s.

sudo fdisk -l gives out this:

Disk /dev/sda: 160.0 GB, 160041885696 bytes
255 heads, 63 sectors/track, 19457 cylinders, total 312581808 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: 0xa42d04a3
 Device Boot      Start         End      Blocks   Id  System
/dev/sda1              63       80324       40131   de  Dell Utility
/dev/sda2   *       80325   102481919    51200797+   7  HPFS/NTFS/exFAT
/dev/sda3       263874558   312580095    24352769    5  Extended
/dev/sda4       102481920   263872511    80695296    7  HPFS/NTFS/exFAT
/dev/sda5       263874560   310505471    23315456   83  Linux
/dev/sda6       310507520   312580095     1036288   82  Linux swap / Solaris

Partition table entries are not in disk order

Disk /dev/sdc: 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 / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x5822aaea

 Device Boot      Start         End      Blocks   Id  System
/dev/sdc1            2048   976769023   488383488    7  HPFS/NTFS/exFAT

The part below "Partition table entries are not in disk order" takes about 5 minutes to appear.

The outputs of ls /dev/ | grep sd before and after connecting the HDD:

before:

sda
sda1
sda2
sda3
sda4
sda5
sda6

,after:

sda
sda1
sda2
sda3
sda4
sda5
sda6
sdd
sdd1

The second output has the lines sdd and sdd1 different from the first one.

IT SHOWED THE FILES!!

The command sudo mount /dev/sdd1 /mnt worked after I typed in sudo fdisk -l!!! Thanks a million!! :) :)