Architecture Of Hard Disk

What Is a Hard Disk Drive?

Your Hard Disk or sometimes called a Hard Drive, is the main storage space inside your PC. It is a permanent storage component unlike the RAM (Random Access Memory). It is possible for your computer to function without a hard disk, however it would be basically useless to you as you would not be able to use an Operating system or have any programs to use. Unless you had another form of mass storage device such as an SSD (Solid State Drive).

Hard disks use circular hard platters to store data on. They are in pristine condition with a mirror like finish to them. These platters are locked away inside a steel casing as unclean air can easily ruin a hard disk. This is why you should never remove the casing from the hard disk, it is very unlikely you will be able to put it back together as a working component. Even a small amount of dust can render a hard disk platter useless.

Below you can see a labeled diagram of a hard disk. The model is a SCSI (Small Computer Scientific Interface) You can see the hard platters on top of each other with a set of arms which hold the read/write head. The speed of the arm is truly amazing as well as the accuracy of the head which can read and write to perfection on a platter which is rotating around 7200RPM. The hard disk looks a very simple idea and probably is, however a lot goes on before the simple writing to the disk its self. We will explain a little more later in the article.

How does the hard disk store data?

On each of the platters there is a thin layer of magnetic film. Data storage on hard disks is very similar to that of a cassette tape. Data is stored in many 1's and 0's stored in different directions on the magnetic film by using a very fine oxide. These binary digits are arranged in different ways to represent different characters. When these are read back by the head the data is retrieved and processed. because no physical contact with the platters is made the disk can be re-written time and time again without damage to the disk or its platters.

File Systems

A file system is the way in which your computer stores data on the hard disk. The most common file systems are FAT16 for older computers, FAT32 and NTFS. FAT stands for File Allocation Table. NTFS stands for NT FileSystem. Both have advantages and disadvantages. FAT16 was a very limited file system in the way that it would store data very in-efficiently, every file would take up a minimum of 32Kb in space as this was the minimum cluster size in a FAT16 system. Also it was only capable of using hard disks up to 2Gb in size. FAT32 solved this problem by reducing the cluster size to 4kb which saved a lot of wasted space and also allowed disk sizes up to 2 Terra bytes. NTFS is believed to be a far greater file systems than any of the FAT's. The cluster sizes can be altered to anything as low as 512bytes which means almost no wasted space on the hard disk. The maximum disk size is a unbelievable 256 Terra bytes, which is very big !!!. NTFS also has added security for file loss.

Measuring the Speed of a Hard Disk

There are various ways of measuring the speed of the hard disk. The main ones are the maximum data transfer rate, the spindle rotation speed and the seek time.

Maximum Transfer Rate - This is the highest amount of data that can be transferred per second. Common forms of hard disks come with an ATA format. the speed rating of an ATA100 disk would be 100Mb/s. Likewise a ATA66 disk would be able to transfer a maximum of 66Mb/s. Past the older ATA standard just mentioned comes the newer S-ATA standard (serial- ATA). S-ATA 1.0 transfers at a max rate of 1.5Gb/s S-ATA 2.0 transfers at a max rate of 3Gb/s and S-ATA 3.0 can transfer data a maximum rate of 6Gb/s

Spindle Rotation Speed - The rotation speed of the disk really is the basis of the other two factors of hard disk speed. The faster the rotation speed, the more data can be written per second and the quicker it is to find the correct data on the platter. A Common speed is 7200RPM (revolutions per minute)

Seek Time - The seek time of a hard disk is the average time it takes for the disk to find the data you need on the platters. A fast spinning, highly accurate and responsive disk will have a shorter seek time and will perform much better, especially when the data is scattered around the disk. Seek time is measured in milliseconds.

Why a hard drive has less storage space than promised?

It has happened to most of us. We buy a new hard drive (or maybe a flash drive) with mind boggling storage capacity only to find that it has less space than what was mentioned on the box. Angered, we start cursing the manufacturer and our dealer for false marketing thinking that they should be sued for doing this. Hey, but have you ever wondered how they continue to do this again and again without getting into legal trouble?

The answer is that they are not marketing it falsely at all. Surprised? I'll explain.

A manufacturer considers 1 Megabyte to be 1000 Kilobytes, 1 Gigabyte to be 1000 Megabytes, 1 Terabyte to be 1000 Gigabytes and so on. This is correct considering that kilo means 1000 and mega means 1000000 (10^6). However, computers calculate on base 2 and to them, 1 MB is actually 1024 kilobytes, 1GB is 1024MB and 1 TB is 1024GB. This difference in the method of computation is responsible for this "missing space."

Lets take an example of a 500 GB hard disk.

From a manufacturer's point of view, the 500GB will have 500*1000*1000*1000 = 500000000000 bytes.

From a computer's point of view, 500GB is actually 500*1024*1024*1024 = 536870912000 bytes.

So, a hard drive that promises to have 500 GB storage space will actually display 465.66GB, 536870912000-500000000000 = 36870912000 bytes (34.34GB) less storage space when connected to a computer.

Space Promised	Displayed on a computer	Difference
100GB	93.13GB	6.87GB
250GB	232.83GB	17.17GB
500GB	465.66GB	34.34GB
1TB	931.32GB	92.68GB
2TB	1862.64GB	185.36GB

Take a look at the table given above to see how much space is "lost" due to computers working on a base 2 system. As you can see, with the increase in capacity of the storage device, there is an increase in the missing space.