Your computer uses two different types of storage devices: internal memory and external memory. While both types save and access your data files, they do so in different ways. Internal and external storage devices have very different physical and operational characteristics.
The two broad categories of computer memory are random-access memory and read-only memory. When you run a program, it stores its processes in RAM. RAM is volatile, which means if you close the program without saving a file, you lose whatever changes you made since the last time you saved the file. If you saved the file, the computer commits the data to ROM, which is not volatile, so even if you turn off the computer, you will not lose the data. Internal memory consists of both RAM and ROM, while external devices use only ROM.
Internal memory is limited in its versatility. Generally, once it's connected to a computer, that's where it stays. You cannot remove internal memory without taking your computer apart. However, as you may expect, you don't have to keep external memory attached to one computer. External storage devices are designed so you can easily share these devices between computers.
Computers ship with a finite amount of internal memory. You may see the storage capacity advertised along with the purchase price, but some of internal memory, or system memory, is reserved for the operating system and applications currently running on the computer. The only way to increase internal memory is by replacing the internal memory chips with larger-capacity modules or by installing new memory chips in available memory-expansion slots, if the system's motherboard supports them. RAM is also used to store the files that define the file system structure to allow the computer to read and write to its devices properly.
Internal memory is usually chips or modules that you attach directly to the motherboard. Internal ROM is a circular disc that continuously rotates as the computer accesses its data. External memory often comes in the form of USB flash drives; CD, DVD, and other optical discs; and portable hard drives.
- Jeffrey Hamilton/Digital Vision/Getty Images