Data storage hardware. View of several forms of computer data storage disk drives. They are, from top to bottom: a zip drive, an external hard disk, a CD-ROM player, a SyQuest removable drive and the CPU (central processing unit), with a removable floppy disk (black) popped out of the drive. There is a modem to the left of the CD-ROM drive. Modems are devices for connecting computers through existing telephone lines. The data storage drives seen here all act as extra memory for the computer. The CD-ROM (compact disc read-only memory) is the only one of these in which the data cannot be altered. The other drives can have new data written onto them by the user.