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.

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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?

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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.

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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.

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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.

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IT SHOWED THE FILES!!

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