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In a computer, data sent to and read from the hard disk is initially read by the hard disk controller. The hard disk controller gives the processor access to read, write, and modify data on the hard disk, CD-ROM, flash drive, and any other storage device. If the operating system needs to read or write data to a storage device, it examines the storage device to determine where to read from (file location) or where to write to. After a read/write location is obtained, the hard disk controller tells the drive to read or write.
All data stored on a hard disk is done magnetically. If the computer needs to read data from the hard disk, it reads magnetic polarities from the hard disk. One side of the polarity is 0 (off) and the other side is 1 (on). The computer reads the 0 or 1 as binary data. Each 0 or 1 is referred to as a bit (a contraction of binary digit). This means that the computer reads data from its storage location as a binary number. For the computer to write information to the hard disk, the magnetic polarities are aligned with the hard disk’s read/write head. The read/write head then writes 0s and 1s that can be read by the computer at a later time. This means that the computer writes information to the hard disk as a binary number.
As mentioned above, a bit is a unit of digital data, containing a single value of either a 0 or a 1. Digital information can be very large, and thus can result in incredibly lengthy strings of 0s and 1s. A string of eight bits is known as a byte. So a byte is also a unit of digital information, and it contains eight bits.
Similar to RAM, storage space is measured in bytes. As the capacities of digital devices grew, new terms were developed to identify the capacities of processors, memory, and disk storage space. Prefixes were applied to the word byte to represent different orders of magnitude. Since these are digital specifications, the prefixes were originally meant to represent multiples of 1024 (which is two raised to the 10th power), but have more recently been rounded to mean multiples of 1000. The table below shows various storage sizes, how they are related to bytes, and an example of media with an approximate storage size.
| Prefix | Represents | Example | Storage Device | Value Contained |
|---|---|---|---|---|
| B | Byte | Byte = Eight Bits | 1 Character; e.g. the letter “t” is 1 byte | |
| KB | Kilobyte | 1 KB = 1,000 Bytes | Two or three paragraphs of text | |
| MB | Megabyte | 1 MB = 1 Million Bytes | Floppy Disk |
One-minute MP3 file One digital picture (four megapixel) Four books (200 pages each) |
| GB | Gigabyte | 1 GB = 1 Billion Bytes |
DVD-R Blu Ray Disc Hard Disk USB Flash Drive |
One 650MB CD of data 256 MP3 songs 340 digital pictures 600 web pages 4,470 books (200 pages each) |
| TB | Terabyte | 1 TB = 1 Trillion Bytes | Hard Disk |
1600 CDs of data 230 DVDs 40 Blu-Ray Discs 262,100 MP3 songs 349,500 digital pictures 655,300 web pages 4,500,000 books (200 pages each) |
Various storage media have their own storage capacity limitations.
| Storage Media | Description |
|---|---|
| Hard Drive/Hard Disk | Most of today’s personal computers use a hard disk for long-term data storage. Maximum capacity is currently around 2TB. |
| Solid-State Drives | The solid-state drive (SSD) performs the same function as a hard disk: long-term storage. Maximum capacity is currently around 1 TB. |
| Flash Drives | The USB flash drive serves as a portable and removable storage solution. Maximum capacity is currently around 256GB. |
| Optical Disc (CD/DVD-ROM) | An optical disc is another form of portable and removable storage. Maximum capacity is currently 900MB for CDs and 50GB for DVDs. |
Source: Derived from Chapter 2 of “Information Systems for Business and Beyond” by David T. Bourgeois. Some sections removed for brevity. https://www.saylor.org/site/textbooks/Information%20Systems%20for%20Business%20and%20Beyond/Textbook.htm
