The Hard Drive is one of the core hardware components for the computer. It is the component that has Windows 10 stored on it. Windows 10 is an Operating System, which means it operates the system (computer) so that the hardware (i.e Printer) and software (i.e Microsoft Office) can talk to each other to produce a result (i.e a printed documented).
Fig 1.0 - The outside of a computer hard drive
Windows 10 provides a piece of software known as Internet Explorer that allows you to communicate with people over the Internet and view websites. This is made possible because Windows 10 allows a Modem to dial the Internet. Once connected to the Internet it then allows Internet Explorer to see website pages as text and pictures, using different typefaces (fonts) and languages in many cases. Windows 10 has many other technical jobs to do, but to keep it simple just look at it as the thing that allows you to type letters, view websites, communicate with people, listen to music and so on.
With the above said. there is only one thing to remember about the hard drive and that is that the computer will not work unless it has an operating system (such as Windows 10) stored on its hard drive. Furthermore, if the hard drive gets worn out or damaged Windows 10 will become corrupt and/or unusable. And even if the hard drive is brand new it is still possible to damage/corrupt Windows 10 by deleting its files, catching a virus or what ever - So look after Windows 10 and the hard drive it is stored on.
Fig 1.0 - The inside of a computer hard drive. The needle reads/writes data.
When you install Windows 10 it tries to detect the hardware inside the computer. For each piece of hardware it detects (i.e Sound Card) it tries to install its own software for that hardware. If it can not find any of its own software to install for the hardware it will ask you for the CD that came with the hardware, so it can install the software on that CD in order to make the hardware work - So always keep the original CDs/DVDs. If the hardware did not come with a CD/DVD you will need to contact your computer's retailer or manufacturer for a replacement CD/DVD or get the necessary software from their Internet website if they have one.
Once Windows 10 has been installed, with all your hardware detected and installed, you then install your additional software. Additional software means Office, Anti-Virus, Printer, Scanner, Music and Skype software for example. Installing Windows 10, and all the common (additional) software, should take up approximately 50 GigaBytes of hard drive space. This means if you have a 500 GB (500 GigaBytes) hard drive you will be left with 450 GB for your own use - You could install more additional software like Games, store your own Folders and Files, download files from the Internet, store CD/DVD contents and so on.
Each time you go on the internet Windows 10, Internet Explorer and Websites save certain information about you and your activities. For example: Windows 10 might save your User Names and Passwords so you do not have to retype them every time you want to go on the internet. Website pages (text and/or pictures) might be saved so that when you want to look at a certain page again it appears instantly, because Internet Explorer will display the saved page first - If the page is updated by the owner Internet Explorer will then download and display the updated page, if it has a connection to the internet. By saving pages it means you can view those pages at your leisure when you are not connected to the internet. And as most pages stay the same (with the same text and pictures on them) the downloading of updated pages is minimal. The saving of information is done for every website you visit.
On top of this Windows 10 is always saving Settings information, Email information, File information and so on. This is necessary to make your experience of the computer faster and better. For example: When you add or delete an e-mail address to/from your contacts list that contacts list needs to be re-saved. Otherwise you would have to manually type in an e-mail address all the time, as opposed to picking it from the saved contacts list. When you update software, like Anti-Virus and Windows 10 software, the updated files have to be saved onto the hard drive as well.
So with forever growing information, installation files and updated files the computer will realistically need 80 GigaBytes of hard drive space - This is a normal scenario for most people. With the remaining space people either leave it empty or use it as storage for their music files for example. However, as Windows 10 indexes each file it stores on the hard drive, so it can find a file quicker, more hard drive space will be needed for the index itself.
Try to avoid saving redundant files on the hard drive. For example: When you save an installation file, such as a downloaded Game.exe file, onto the hard drive and then install it it will take up additional hard drive space. This is because files that need to be installed have usually been compressed (shrunk). So 10 game files (not 10 games) for example might of been shrunk into 1 small installation (Game.exe) file.
Lets say the 10 game files were 1 MegaByte each (so 10 MegaBytes) and then shrunk into 1 small (3 MegaBytes) installation (Game.exe) file, so it is quicker to download and/or store somewhere. When you activate the small installation file it is expanded back to its original 10 game files, which are then saved inside a Games folder on the hard drive. What this means is you have 10 installed game files and 1 small installation file on the hard drive, so the small installation file is then a redundant file. By saving the small installation file onto CD/DVD for example and then activating it from the CD/DVD you will save yourself 3 MegaBytes of hard drive space. If you now imagine you had 30 small installation files that would be a saving of 90 MegaBytes, if they were on a CD/DVD instead of the hard drive. Of course, these days the 'average person' will never use 500 GB of hard drive space. So to some degree you should not worry about not having enough hard drive space.
Years ago, if your hard drive was 20 GigaBytes or less it would spin round at 5400 rpm (revolutions per minutes). And if it was more than 20 GB but less than 80 GB it would spin round at 7200 rpm. Larger sized hard drives would spin round at 10,000 rpm. These days most computers have a 5400 rpm hard drive installed.
A hard drive is like a record player - It spins round and a needle (laser) reads/writes the data (Fig 1.1 above). The faster the rpm the faster the hard drive. If you have a 5400 rpm hard drive it is definitely worth upgrading to a 7200 rpm hard drive because the speed difference is noticeable - Data is read/written faster. Better still, upgrade to a newer SSD (Solid State Drive) hard drive because they use memory to store data, are much faster and have no moving parts such as a needle; so should be less prone to natural wear and tear and common 'hard drive failures'. One thing to remember though is that when you buy a new hard drive it will be blank - It will have no software installed on it at all. Not even Windows 10.
In terms of hard drive size, although 1 GigaByte can store hundreds of files, you have to look at the size of the average file these days to put hard drive size into perspective. In other words, 500 GB should easily satisfy the 'average user', but 'music lovers', 'video lovers' and 'file downloaders' should consider a larger hard drive. With music downloading, digital cameras, scanners and larger photo files becoming 'the norm' the need for storage space will grow. Hence why many laptops for example come with a 500 GB or 1,000 GB hard drive as standard.
When a computer and its hard drive are new you hear nothing but the fan inside the computer. A humming sound. As the hard drive gets older it starts to make a light jingling/drumming sound that overtakes the sound of the fan. Nothing serious, perhaps a few decibels only. And when the hard drive is really old it makes a noticeable, loudish, screeching/drumming/tapping sound. This is when it is time to replace the hard drive.
As a rule, try and replace the hard drive every 3 or 4 years (5 Maximum). If the hard drive is 160 GB or less it should be in the dustbin already. By not replacing the hard drive you risk losing/damaging your files due to the hard drive's age (Corruption, Unable to read/write data and so on). Also backup (save/copy) your files onto a CD/DVD, Memory Drive and/or USB Hard Drive at least once a month, if not every week, regardless if the hard drive is new or old.