What Is Important Properties Of Hard Disk?

The most important property of any hard drive (not just hard disk, I'll get to that in a sec) is that it has a series of 'toggle switches'.  Think of it like a big wall, covered in light switches, and you read patterns based on which switches are up and which are down. This sounds very complicated, but it's not, because of a way of dealing with numbers called 'binary', where there are only two stages of a number and everything else happens because of where the number (a '1' or a '0') is in the order it is read.

this is how your data is stored on your hard disc, and on your thumb drive, or on your cell phone or in your email account: It all boils down to 1's and 0's.

so the important part of a hard disc, the actual platter that stores the data (it looks like a smaller CD, but a bit thicker and made of glass) is that it can store 'up' and 'down' on the light switches very easy. There are a few technologies to do this, the most common I think (somebody correct me if I'm wrong) is by reading the magnetism and by making small pieces of the disc point one way or the other. If an electronic magnet picks up a high point where its a 'north' signal (remember magnets have two sides), it reads out a '1' if it picks up a low point with a 'south' signal, it reads out a '0'.  I worry that I'm getting the actual reading method wrong here, so go and double check this by yourself if it matters to you, but the principle of it is the same- one part up, one part down, a way of telling, and a system of logic to understand what they all mean.

if you really want to know about the theory of this, I suggest looking up 'state machines' and other logical devices from the very early ages of computing, it's truly fascinating stuff, the time where the mechanical became electronic and then became microscopic.