Disk size question - the vanishing 500 gigabytes

I recently bought a new Iomega external hard disk. According to the label on the box, it’s 2 TB. I just plugged it in and it worked. Fine.
DO9 recognizes it as a disk of 1.8 TB total size. Due to (I think) cluster size, this 'loss' is normal. Anyway, I would be very happy if I had 1.8 TB available.
The problem is that, starting from a disk ‘out of the box’, I have only 1.36 TB available, according to DO9 and according to ‘reality’. So, some 500 gigabyte are ‘somewhere’.

Question: is there some way I can have the ‘full’ 2 TB (or 1.8 TB) available, instead of the ‘mere’ 1.36 TB?

I am using Windows Vista Home Premium. I tried various USB ports, but the results are the same. The file system is NTFS.
I noticed that another external hard disk of 1.5 TB also had only 1.36 TB in reality. So, I had the feeling that both disks are somehow considered as having the same size – even though they are clearly different.

Most probably this is not the proper forum to post this question, but I am a bit at a loss as to where I could get a decent reply. So, I’m just hoping someone out here could guide me a bit.

Could it be partitioned in some way?

What does it look like in "Computer Management" under "Disk Management"? (Sorry, forgotten how to get at that bit in Vista, in Win 7 you can just type "Computer Management" into the start menu search.)

Is there a lump of "empty space" at the end?

Remember that HDD manufacturers mean 1,000,000,000,000 bytes when they say 1TB, while Windows and most Windows software uses 1024 * 1024 * 1024 * 1024 = 1,099,511,627,776 bytes when they say 1TB, so with a 2TB drive that's almost 200GB lost right there.

The other space could be lost to formatting, plus space that the OS may be reserving/using for things such as system restore or volume shadow copies ("Previous Versions").

It's worth checking that all of the space on the drive is actually partitioned, though, as Michael suggests. (As well as space being unused, it's also possible there's a hidden partition with emergency backup utilities on it or something.)

Thanks for the helpful information, Michael and Leo. I'll see how I can put it to use now.