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What is a Hard Disk?

By A. Leverkuhn
Updated May 16, 2024
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A hard disk is a metal plate with magnetic surfaces, and it is a component of a hard disk drive. The hard drive equipment reads from and writes to the surface of the disk. This type of drive is common in most personal computers, as well as other large devices that include data storage.

In a hard drive, an actuator arm assembly locates platter cylinders so that it can read from and write to the disk. Read and write heads act as the agents for these functions. The total assembly fits into a rectangular metal container. In traditional computing, the hard disk was the internal structure for storing data in a workstation or personal computer. External disks were called “floppy disks” because they had soft internal media. The emergence of new types of drives has made floppy disk technology largely obsolete.

Today, a hard disk can be either part of an internal disk drive, or an external drive that connects to a computer. Internal and external drives provide a common method for storing a lot of executable programs. They also provide a stable way to hold many gigabytes of data in a separate drive that can be disconnected from a laptop or desktop computer. An innovation called Perpendicular Magnetic Recording (PMR) is increasing the amount of data that a hard drive can hold.

Hard drives are getting competition from other new technologies, including the flash drive. A flash drive is a solid state drive, where a new form of data storage replaces the circular hard disk technology. The hard drive and the solid state drive often share the same USB connections to the computer, but the way data is recorded and stored is different.

Periodically, a hard drive needs to be defragmented, where the computer re-organizes all of what is written on the drive surface for optimized performance. Because a solid state drive doesn’t write to a disk, it does not need to be defragmented. Solid state drives can be more expensive than hard drives, but they are becoming a more common way to hold and deliver data. Many manufacturers are using solid state drive technology for smaller electronic devices as well as ruggedized computers and some netbooks.

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Discussion Comments

By hanley79 — On Jun 17, 2011

@TheGraham - Installing and uninstalling software can also fragment your hard disk. I managed to fragment the data on my brand new Seagate hard disk by accident by installing a copy of the Poser program, a 3D modeling art program, and then adding lots of the buyable content all at once.

See, Poser's main directory, called a Runtime, holds a bunch of folders with third party 3D models and other content. Installing each and every file over again is a serious pain, so when I do the period blanking of my hard drive and reinstalling everything (it's good for your computer -- I do it every three months), I usually just back up the whole Runtime and then paste it back on after I've reinstalled everything.

Well, as it turns out, copying a huge directory of installed files for one particular program like this seriously fragments your hard disk's data. I didn't defragment before I copied the Runtime to the back up area, so it was probably already a mess and then moving it made it even more fragmented -- and then moving it back fragmented it even more! It took hours of defrag to fix.

Installing software can also fragment, because the software installs the same way as files write to the hard disk -- they write to the nearest available space, then the next, then the next, until they're all there but in fragments.

It's good to defrag before and after any large file movements, whether it's a software installation, software removal, or you're just shifting files around. Even your backups!

By TheGraham — On Jun 15, 2011

@gimbell - Nice run-down on how hard drives write data, and how defrag helps fix fragments. You forgot to mention that fragmentation can also happen by accident, though.

See, it's true that when you save a file to your hard disk, it's saved in the first available chunk of empty space, then the next space, then the next, until the whole file is saved. That's fragmentation on purpose, though.

The fragmentation that most commonly becomes a problem is when somebody cuts or copies files from one place to another in large amounts. Moving around files that are already in pieces might shift some of the pieces into two different places, themselves, further fragmenting the file.

So, moving files around a bunch can fragment them. And if you've ever backed up files a bunch, you know where this is leading... Backing up your files fragments them!

To minimize the defragmentation going on, it's best to always defragment your hard disk before you save a large amount of files over to a back up location. This way they are saved in bigger chunks so that they don't get even more fragmented when you save them back to your computer.

By gimbell — On Jun 14, 2011

@robbie21 - VivAnne noted what happens if you don't use defrag, and so pointed out that it's necessary, but didn't really explain why. To address your question of whether new computers need to have defrag run on them in a bit more depth, I wanted to add that defrag doesn't get outdated because it still applies to our technology.

Hard disks, especially IDE hard disks, write by etching tiny marks on the actual disk in arrangements of only two marks that represent the 0s and 1s found in binary. The data can later be read by the computer spinning the disk, much like a CD-ROM, and scanning over the marks.

All of that applies to hard drives and defrag because hard drives, when they save a file, write data in the first available spot. If that spot isn't big enough, when it's full they hop to the next available spot to write the rest. That means some files that load as a full file on your computer are actually saved in pieces in different spots -- fragments.

What defrag does is rearranges where file fragments are saved. Specifically, it does its best to move files that are cut into fragments to the same spot on the hard drive so that the files are whole instead of fragmented. This is why it is called defrag, short for defragmentation.

So in conclusion, as long as hard drives save data in this way -- in the first available area -- and as long as data is saved by etching tiny marks, defrag will still be necessary and helpful.

By ahain — On Jun 12, 2011

@VivAnne - Good points all -- especially the tip with the search box for defrag. With Windows, the search box is really a sure-fire way to find any program, as long as you know the name of it, and in Windows 7 the search box can even find words and phrases that are inside of files such as text files, so it searches even better.

By VivAnne — On Jun 11, 2011

@robbie21 - I just addressed this issue with an online friend of mine the other day whose computer was nearly tens years old, and as he put it, "a dinosaur". Since it is a laptop, I would imagine it's pretty chunky in design, but since I oly know him online I've never actually seen the thing.

Anyway. he had not only never backed up the laptop hard disk, including dozens of fiction stories because he is a hobbyist fiction writer, but he also had never even heard of defrag. He had never defragged the computer in all of the years he had owned it!

I walkedhim through where to find defrag, and when he scanned his drive it was a whopping 80% fragmented. He had to leave it on defrag literally overnight to give it enough time to work all of those file fragments into file blocks, but defrag did the job, and now his computer runs much better (even though it's a dinosaur.)

While walking him through how to defrag, I used defrag on my own two computers -- a desktop computer with Windows XP and a Netbook with Windows 7 Starter. Both of them were fragmented enough to need to run the program, so I can safely say that defrag is very useful and necessary even on newer computers.

As for where to find it, just go to your Start menu, Programs, Accessories, and it's under the System Tools folder. That's in Windows XP -- when in doubt, just type "defrag" into the search and it will come up.

By allenJo — On Jun 11, 2011

@NathanG - Hard disk technology has its limitations, but it’s still durable in my opinion. I have a USB hard disk in addition to my internal hard drives. The USB disk is an external hard drive and I use it for my backups. Some people use USB flash drives, but I’d stay away from them.

From what I understand, the flash drives are more susceptible to crashes than regular hard disks. I think the memory gets corrupted more easily or something like that.

Just make frequent backups of your hard disk, and you should be good.

By NathanG — On Jun 11, 2011

@everetra - A good way to keep your disk from crashing is to wipe the hard disk clean now and then. There’s nothing like the complete reformatting of your hard drive to give you a second lease on life.

Of course, this will not do much if the hard disk failure is related to something wrong with your boot sector. It will only work if it is software related-your operating system got corrupted or something like that. You could always reinstall your operating system, but I think a complete wipeout is a cleaner option. You leave nothing to chance that way.

By everetra — On Jun 11, 2011

@miriam98 - If your disk does crash, there are some hard disk repair programs you can find online that claim to recover the data. I’ve never used them, and I’ve read mixed reviews about whether they work.

There are also services out there that will charge to recover your data, for a pretty penny. I’ve read that when your data disappears that it never actually gets deleted-it just gets stored somewhere else, and this is the premise behind disk recovery.

I’ve never used them myself however. I did have a disk crash once and took it in for repair-from a regular computer shop, not a data recovery service. The shop could do nothing to get my data back. I just try to make backups of my personal stuff now and then.

By miriam98 — On Jun 11, 2011

Hard disk failure is a fact of life. That’s something I’ve learned over and over again; sometimes I had backups, many times I didn’t. You’d think once would cure you.

However, one thing I’ve found lately is that the dreaded day does not have to come upon you unawares. You can perform a disk check on a periodic basis in Windows and it will correct any bad sectors that it finds, and keep your disk running error free.

There are also some software programs on the Internet that claim to be disk doctor type applications, which will monitor the overall state of your hard disk and let you know before something bad happens.

It’s nice to have advanced warning, instead of waiting until that awful day when you hear that grinding sound and realize your data is gone forever.

By robbie21 — On Jun 11, 2011

Does anyone else remember the old days when a hard drive was forty megabytes--and that was cutting edge? Every time I download an iTunes update or something like that and it's eighteen or twenty-five megs or whatever, just for a second it seems enormous!

We had top-grade computers at my house growing up, but my dad had this old AT&T telephone, one of those big black ones. My mom once caused a hard drive failure by putting it on top of her CPU. (Yeah, remember when those were so big you use them like furniture?) The magnet inside erased it!

I do remember defragging my hard drive as a kid, but do you have to do that with the newer computer? I know my Mac didn't even come with a way to do it.

By jennythelib — On Jun 11, 2011

@anon112180 - That's going to vary a lot, depending on what kind of computer the hard disk is for, whether it's internal or external, and how big you need it to be. You can find good prices at online vendors, but make sure to check out the site before you purchase because there are scam sites out there. (Look for reviews of the site or go with something you've heard of.)

By anon112180 — On Sep 19, 2010

what are the prices of hard disks?

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