Computer Storage Devices

Before you start

Objectives: Learn which different storage devices are there and which technologies they are using.

Prerequisites: no prerequisites.

Key terms: disk, drive, hard, data, flash, tape, memory, surface, information, floppy, read, access, head, optical, platter, ram, dvd, cd

Hard Disk Drive

A hard disk is a magnetic disk encased in a thick protective shell. Hard disk drives can store huge amounts of information, and they’re relatively fast. They are produced in an clean environment. The air inside of the hard disk drive is also clean. If we open it up and allow regular air inside, we’re going to destroy the drive. We should only open drives that we don’t intend to keep. Inside the hard disk drive are a series of platters. These are aluminum disks that are coated with a magnetic material that can be used to store data. There is also an Actuator Arm with head on it. The head is used to read and write information from the surface of the platter of the hard disk drive. The head on a hard disk drive does not touch the surface of the hard disk and that’s because when the disk spins, it creates what’s called a Bernoulli Effect. The spinning of the disk creates a small cushion of air between the surface of the platter and the surface of the head. It’s close enough that the magnetism in the disk or the magnetism in the head can magnetize or demagnetize a certain area of the hard disk drive to store our binary data, but they don’t actually touch.

Hard drive

 Image 325.1 IDE Hard Disk Drive

A hard disk consists of several platters. Each platter is double-sided, meaning that we can read and write data from the top and the bottom. Each platter has its own set of read-write heads, one on the top and one on the bottom. All of the read/write heads are attached to a single access arm to prevent them from moving independently. Each platter has track and track locations that cuts across the platters called a cylinder.

The advantage of a hard disk drive is that it’s relatively fast. An inexpensive hard disk drive will turn around 5,400 RPMs. A good quality hard disk drive will do it at 7,200 RPMs. And a high end drive will be 10,000 RPMs or faster. The faster this disk turns, the more data that the head can read and write within a given amount of time. Another advantage is capacity. Hard disk drive can store gigabytes or even terabytes of data. Bigger and bigger hard drives are being made available each year, and the cost per MB is getting cheaper. The one disadvantage is portability. It’s very difficult to take our hard disk out and move it to another computer. However, there are portable versions available that use a USB or FireWire interface. That way we can unplug the hard disk drive from the USB port on one system and plug it in into the USB port on another system. One other problem is that they are prone to failure (bad sectors). They are also vulnerable to physical damage, for example when dropped.

Optical Drive

An optical drive can be CD or a DVD. An optical drive works in a different manner than a hard disk drive. With a hard disk drive we use rewrite heads that are essentially magnets to magnetize or demagnetize areas on a surface of a platter in a hard disk drive. Optical drives do not use magnetism. Instead, they use light impulses to encode binary data. If we look at the surface of a CD or DVD media with a powerful microscope, we will see something like this.

 CD Surface

Image 325.2 CD Surface

As you can see we have pits and lands on the surface. These are physical etchings. This creates a system where certain areas of this disk reflect light, and where other areas do not reflect light. Imagine that the disk is spinning and that the laser light is hitting those lands and pits. The laser light would sometimes reflect, and sometimes wouldn’t. By doing this, we can construct binary data by converting those light impulses. If we see an impulse, it’s a 1. If we don’t see an impulse, it’s a 0 (or vice versa).

The advantage of optical drives is that the media is very portable. We can take a CD or DVD disk from one system to another and very easily transfer data. They also store a fair amount of data. A CD will hold 680 MB or 700 MB of data. A single-layer DVD will hold 4.7 gigabytes, and a double-layer DVD will hold twice that amount of data. One of the issues with an optical drive is that the data is either permanent or semi-permanent. Permanent means that it’s a read-only. We call them CD-ROMs which stands for CD-Read Only Memory. There are also rewritable versions of CD or DVD, where we can physically rewrite the data that’s on the disk. One of the problems with optical media is that they are susceptible to damage. The surface of the disk can get scratched in such a manned that it’s no longer readable. It can be a very frustrating thing.

Flash Devices

Flash devices are storage devices that use memory chips which are programmable and non-volatile (persistent). The memory chips inside of a flash device are very different than the memory that’s inside of a RAM chip. Remember, with RAM we have to have a constant supply of power. If we don’t have that, the memory chips in RAM will lose their contents. That is not the case with a flash device. This type of memory is re-programmable, which means that we can write information to the memory locations, remove power, and the contents of the chips will remain intact. That enables us to create devices that will function basically as a hard disk drive. We can store and read data just like we would with a hard disk drive. The only difference is that instead of writing data to a spinning platter, we’re writing data to a memory location in a memory chip.

The advantage of flash devices is that they are very portable. We can take a flash drive and move it from one system to another. It is also relatively fast. The capacity of flash devices is getting bigger and bigger. One of the key things is that they are rewritable. Just as we can rewrite information on a hard disk drive, we can just as easily rewrite information on a flash device. They are also very small. Flash drives come in a variety of different types and different interfaces. For example, we have a thumb drive which is a flash drive on a USB port. We can plug it into a PC and access the memory locations in the thumb drive through USB connection. Digital cameras can use other types of flash memory such as CF, MMC and SD memory. These are also flash devices that store data. The bad thing is that different card formats require different readers.

Solid State Drive

Solid state drives (SSD) also uses flash technology, but instead of being designed for a portable storage solution, it’s designed to take the place of a hard disk drive. The advantages of SSD is that it’s a lot lighter, and it doesn’t create as much heat.  It consumes less energy than the standard hard disk drive, so that makes it good for laptops. They are less susceptible to physical damage (from dropping) and immune from magnetic fields. The main disadvantage currently for solid state drives is cost. They are several times more expensive than comparable hard drives.


Image 325.3 – SSD

Tape Drive

Tape drives are also called Digital Linear Tape or DLT drives. They store data on magnetic tape, similar to audio cassette tapes. Tape drives are most commonly used for backing up data, such as taking a backup of all the hard drives in the system.

The advantage of a tape drive is that it can store large amounts of data. That’s because they are very popular for backing up information from our hard disk drives. The next advantage is that tapes are relatively inexpensive. Another advantage is that they store long-term very well. They do have some disadvantages. The key disadvantage to tape drives is speed. It takes a long time to write information to the tape. The other disadvantage is the way data is accessed on a tape drive. The problem is that it’s not random access. When we access information a DVD disk, or on a hard disk drive, or on a flash drive, it’s random access. That means that on the hard disk drive, we can position the heads over any sector we want immediately. It only takes a few milliseconds to move the heads to the right location and read information from any location on that disk. The same thing is with RAM. We can read any memory location from RAM right away. That’s random access. Tape drives are linear, not random access. That means that if we want to read a file that is located somewhere on the end of the tape, we have to fast forward through the tape until we reach the right location. That takes a lot of time compared to random access. One other disadvantage is the fact that the tapes and the drives themselves tend to wear out over time.

Tape Drive

Image 325.4 – Tape Drive

Floppy Disk

Floppy diskettes are probably the oldest type of storage device in a given PC system. Most PC systems that we purchase today probably won’t have a floppy diskette anymore. The reason we call them floppy disks is because they have a Mylar-coated disk inside of a sheath. There were two types of Floppy disks. There was 3.5 inch. This is the one that we’ll probably be most familiar with. The older version was 5.25 inches. We don’t see those anymore.

 Floppy Disk

Image 325.5 – 3.5 inch diskette

If we split our 3.5 inch diskette in half, inside of the sheath we’ll find a Mylar disk which is coated with a magnetic surface material that allows us to read from and write information to the disk. The 3.5 inch disk is double-sided, and 5.25 inch disk was single-sided. The latest versions of Floppy disk drives have two heads that read and write information at the same time. The heads, access the surface of the Mylar disk through the holes in the sheath. Heads actually make physical contact with the surface of the disk. While that works well, it also wears out the disk very quickly.
The advantage of a floppy disk is that it was portable. It was very easy to take your diskette from one system to another. Also, it can be used as a boot device. The disadvantage of floppy disk is that it had low capacity. The 3.5 inch floppy disk stores 1.44 megabytes of data. That’s not a lot of data in today’s world. The other problem is that they’re slow. The disk only turned about 300 RPMs.

Removable Storage

The term removable storage refers to the ability to easily connect and disconnect storage devices or storage media from a computer, when compared to internal or fixed storage. Floppy disks, optical discs, flash devices, eSATA drives, and tapes are examples of removable media. Hard disks and solid state drives are typically not considered removable media as they are installed internally in the computer (except if we have external hard disk or solid state drive).

Different Interfaces

The interface identifies the physical and electrical properties that allow for the transfer of data. One of the oldest interfaces is parallel ATA, also typically referred to as IDE.


PATA was the original specification that was used for hard disks, CD and DVD drives. PATA is also called EIDE, IDE, and ATAPI. Parallel ATA is a parallel internal interface. In a typical motherboard configuration we have two parallel ATA ports, each that support up to two devices. A single cable can connect two different devices. That means that we can have a hard drive and an optical drive that share the same channel (two devices per channel). Because both devices share the same channel, devices must be configured properly to avoid conflicts. Transfer rate (speed) of parallel ATA has a limitation of 133 MBs.

IDE Cable

Image 325.1 – PATA Cable


Image 325.2 – PATA Port

We have a separate article in which we describe PATA in detail. A second standard for connecting devices is serial ATA or SATA.


Serial ATA is a new specification that is replacing the parallel ATA connection. It is also used for hard disks and optical drives, however it can be internal or external. The external specification is referred to as eSATA. eSATA is faster than USB and Firewire. Serial ATA uses a serial connection. Each device connects through a serial cable to its own port on the motherboard. This allows a dedicated channel between the device and the computer. Serial ATA can transfer data faster than the older parallel ATA standards. Transfer rate of Serial ATA starts at 150 and depending on the version can go up to 300 or even 600 MBs. SATA is hot swappable and provides built-in support for disk protection methods. There is no need for additional configuration of SATA devices, we simply connect the device to the SATA port.

SATA Cable

Image 325.3 – SATA Cable

SATA ports

Image 325.4 – SATA Ports

eSATA Cable and Port

Image 325.5 – eSATA Cable (left) and Port (right)


Another interface that we should be familiar with is SCSI or Small Computer System Interface. SCSI connect multiple devices onto a single bus or chain. For instance we may have 4, or 5 devices connected in a chain to a single port. Is relatively difficult to configure because devices must be configured with a device ID, and the bus must be terminated. SCSI is typically used in high-end workstations or servers (tape storage devices and hard disks), and rarely used in desktop systems. Serial ATA performance is similar or above that of the SCSI interface. SCSI devices can be either internal or external, and in general SCSI is typically more expensive than PATA/SATA.

USB and FireWire

Two other interfaces that are commonly used for storage devices are USB and Firewire. These device interfaces are typically always external. One advantage to these interfaces is that they are plug and play, in that you can connect a device to a port and have it recognized and configured immediately without shutting down your system. They are also hot swappable. We have separate articles about USB interface and about FireWire interface.


With a hard disk drive we use rewrite heads that are essentially magnets to magnetize or demagnetize areas on a surface of a platter in a hard disk drive. Optical drives use light impulses to encode binary data. Flash devices are storage devices that use memory chips which are programmable and non-volatile (persistent). Solid state drives (SSD) also uses flash technology, but it’s designed to take the place of a hard disk drive. Tape drives store data on magnetic tape, similar to audio cassette tapes. Floppy disks are not used any more. For internal storage device we’ll typically use parallel ATA or serial ATA. Serial ATA is newer standard. For external devices we can use a USB or Firewire connection, or we can use an eSATA connection for serial ATA device. SCSI connections are used in servers and are typically not used in desktop systems. USB, Firewire, and serial ATA are hot swappable capable interfaces.