The Hard Drive is one of the core hardware components for the computer. It is the component that has OS X (El Capitan or Sierra) stored on it. OS X (El Capitan or Sierra) is an Apple Mac Operating System, which means it operates the system (computer) so that the hardware (i.e Printer) and software (i.e Microsoft Office 2011) can talk to each other to produce a result (i.e A printed documented).
OS X (El Capitan or Sierra) provides a piece of software known as Safari that allows you to communicate with people over the internet and view websites. This is made possible because OS X (El Capitan or Sierra) allows a Modem to dial the internet. Once connected to the internet it then allows safari to see website pages as text and pictures, using different typefaces (fonts) and languages in many cases.
OS X (El Capitan or Sierra) 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. There is only one thing to remember about the hard drive and that is that it will not work unless it has an OS X operating system, such as El Capitan or Sierra, stored on it. If the hard drive gets worn out or damaged OS X (El Capitan or Sierra) will become corrupt and/or unusable. And even if the hard drive is brand new it's still possible to damage/corrupt OS X (El Capitan or Sierra) by deleting its files, catching a virus or what ever - So look after OS X (El Capitan or Sierra) and the hard drive.
When you install OS X (El Capitan or Sierra) it tries to detect the hardware inside the computer. For each piece of hardware it detects (i.e. the Sound Card) it tries to install its own software for that hardware. If it cannot find any of its own software to install the hardware, it will ask you for the original Installation CD/DVD that came with the hardware; so it can install software from that Installation CD/DVD in order to make the hardware work - So always keep the original Installation CD/DVD that came with the computer and its hardware.
If the hardware did not come with an Installation CD/DVD, you will need to contact your computer retailer or manufacturer for a replacement CD/DVD or get the software from their website if they have one. In the case of OS X El Capitan and Sierra they can be downloaded from the Apple Store.
Once OS X (El Capitan or Sierra) has been installed, with all your hardware detected and installed, you then install your additional software. Additional software means Third-Party software such as Microsoft Office 2011, BitDefender Anti-Virus, Epson All-In-One (Printer, Copier and Scanner), Music Player, Skype Instant Messenger and so on.
Installing OS X (El Capitan or Sierra), and the common (additional/third-party) softwares, should take up no more than 40GB of your hard drive's storage capacity (storage space). This means if you have an hard drive with a storage capacity of 500GB (500 GigaBytes) for example you will be left with at least 460GB for your own use - So you could use that free storage space to install more additional software (such as Games), store personal Folders and Files, download files from the internet, store cd/dvd content and so on.
Each time you go on the internet OS X (El Capitan or Sierra), Safari and Websites save certain information about you and your activities on the hard drive, inside special files. For example: OS X (El Capitan or Sierra) might save your User Names and Passwords so you don't 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 particular website page again it appears instantly, because Safari will display the saved website page first - If the website page is updated by the author Safari will then download and display the new content of that updated website page, if it has a connection to the internet.
By saving website pages it means you can view those website pages at your leisure when you are not connected to the internet. And as most website pages stay the same (with the same text and pictures on them) the downloading of updated website pages is minimal. The saving of information is done for every website page you visit.
On top of this OS X (El Capitan or Sierra) is always saving Settings (Preferences) information, E-mail 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 OS X (El Capitan or Sierra) software, the updated files have to be saved onto the hard drive too.
So with forever growing information, installation files and updated files being saved on the hard drive the computer will realistically need at least 60GB (60 GigaBytes) of hard drive storage space to accommodate these files - This is a normal scenario for most people. With the remaining storage space people either leave it empty or use it as storage for their Music files for example. However, as Mountain Lion 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 FREE_Office.pkg 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 App files (not 10 Applications) for example might of been shrunk into 1 small installation file (i.e. FREE_Office.pkg).
Lets say the 10 App files were 1 MegaByte each (so 10 MegaBytes) and then shrunk into 1 small (3 MegaBytes) installation file (i.e. FREE_Office.pkg), so it is quicker to download and/or store somewhere. When you activate that small installation file it is expanded back to its original 10 App files, which are then saved inside a folder named after the application itself (i.e. FREE_OFFICE) on the hard drive. What this means is you now have 10 installed App files and 1 small installation file on the hard drive, which in turn means the small installation file (i.e. FREE_Office.pkg) is then a redundant file.
By saving the small installation file (i.e. FREE_Office.pkg) onto a cd/dvd for example and then activating it from that cd/dvd means you will save yourself 3 MegaBytes of hard drive storage space. If you now imagine you had 30 small installation files saved on a cd/dvd, as opposed to saved on the hard drive, that would be a saving of 90 MegaBytes. With today's bigger hard drives 90 MegaBytes is not really much to worry about, so why mention this storage scenario? Why? Because it's not just about saving on storage. It's also about not letting your anti-virus software and the OS X (El Capitan or Sierra) indexing system scan unnecessary files, therefore avoiding unnecessary wear and tear of your hard drive.
The average Apple Mac computer these days is fitted with an hard drive that has a speed of 5400 RPM - Revolutions (Cycles) Per Minute - unless it was upgraded to a 7200 RPM hard drive for whatever reason(s). The hard drive is like a record player - It's drum (turn-table) spins around while it's needle (laser) reads/writes the data (Fig 1.1 above). The higher the RPM the faster the hard drive drum spins. If you have an old 5400 RPM hard drive, from 2006 for example, it's definitely worth upgrading to a 7200 RPM hard drive simply because the speed difference is noticeable - Data is read/written faster.
One good reason for upgrading is because it doesn't cost that much to do so. The price gap between a 5400 RPM hard drive and a 7200 RPM hard drive these days is minimal. Another good reason is because older 5400 RPM hard drives are becoming obsolete, such as those with a storage capacity of 160GB or less. Saying this; One thing to remember here is that when you buy a new hard drive, of any storage capacity, it will be blank - It will not have any software on it at all. Not even OS X (El Capitan or Sierra).
In terms of hard drive size (storage capacity), although 1 GigaByte is 1,024,000,000 Bytes, you have to look at the size of the average file these days to put hard drive size into perspective. For example - 500GB can store approximately 600 'average-sized' downloaded Movie files, but this depends on the quality and length of each actual movie because the higher the quality (i.e. High Definition 1024) and the longer the movie (in minutes, not file size) the more MegaBytes it will have used. To put this into perspective; A DVD is 4.7GB which means a 500GB hard drive could store up to 100 DVDs in reality (4.7GB x 100 = 470GB).
Although in reality you will never use the whole 500GB, never say never. With music downloading, digital cameras, scanners and bigger files becoming standard the need for storage space will keep growing. Hence why many apple mac laptops for example come with a 500GB 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's time to replace the hard drive.
As a rule, try and replace your hard drive every 4 or 5 years (6 Maximum) because it's around this time that an hard drive naturally suffers from its years of wear and tear. 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) your files onto CDs/DVDs once a month or so, regardless if the hard drive is new or old.
Hard Drives come in all shapes and sizes these days with the USB Hard Drive being a popular choice because of its small physical size and portability. It's a standard hard drive that is encased with a durable, hard, plastic cover that connects to the computer via a USB Cable.
Fig 1.2 The external, SeaGate GoFlex, USB Hard Drive connects to the computer via a USB Cable.
USB Hard Drives are ideal for storing backup copies of your personal files (i.e. music, video, photo and document files) and also for making monthly backups of OS X (El Capitan or Sierra) system files. You could store these backup copies (files) on your built-in apple mac hard drive but if anything goes wrong with that, for whatever reason(s), you will at least have one 'safe copy' of those files on your usb hard drive. So it doesn't matter that your internal, built-in, apple mac hard drive is small because you can backup more data onto a bigger external, usb, hard drive anyway; therefore relieving your internal, built-in, apple mac hard drive of some natural wear and tear.
It's worth noting here that standard usb hard drives, such as the SeaGate GoFlex range of usb hard drives, work perfectly well with both apple mac and windows computers. If you want an hard drive that is apple mac specific, with wireless capability, you should take a look at the Time Capsule.
Fig 1.3 The apple mac Wireless Network controlled external hard drive
As said: The Time Capsule is a Wireless Device that is fitted with an internal hard drive. This means it can transfer and store files via a Wireless Network, as opposed to transferring files via a USB Cable. It can also backup your files via the Time Machine application/feature.
With wireless technology becoming more and more popular and in demand, within the computer and mobile phone markets, I would imagine in the future that even the humble usb hard drive will be obsolete.
At the time of writing the classic internal hard drive, as shown in Figures 1.0 and 1.1 above, are slowly being replaced by SSD (Solid State Drive) hard drives which are hard drives that do not have any moving parts (i.e. the drum and wheel have been replaced by memory blocks). Furthermore, the classic usb socket is being replaced with the newer C-Type USB Socket.