HD manufactures use the base 10 Gigabyte, which is slightly smaller than the base 2 measure software and computers use for data. That's why your HD always looks smaller when you inspect it in windows. For instance, my 160 GB hard drive is 149 GB in windows.
Hey Weebl's stuff, what do you think of Owls?
http://www.weebls-stuff.com/songs/Owls/
Edit: http://www.youtube.com/watch?v=9hBpF_Zj4OA
Indeed.
The hard drives you buy are in GiB as in the exact number of bytes, this was done once as a marketing trick to make it seem you buy more memory, now all company's do it. (as in 1MiB are 1000 Bytes, while 1MB is 1024 Bytes thus your computers gives a smaller number then what the manufacturer says.)
Data is stored in 0 and 1, we all know that, it is deleted by deleting the small piece of information witch tells your computers where exactly on your disc it wrote that information. without that the next time you save something it will just over write your old data. Information is saved in different "blocks", you can choose the size of them from a few Kb to a few MB and you can select the size normaly when formatting a HDD. No matter how small a file is it will always take minimum 1 block of space.
To delete a file there are also different methods, for secure government information it is not good enough to just delete that small file, you can still easily recover it, there they use special software and change all the bits to a 0, or a 1 for example, some software only changes some bits in semi random order, it goes faster but is not as safe.
Hmm, i still remember something from computer class it seems, this is all from my memory folks, i might not be 100% correct but i think i am.