Magnetic tape and disk have evolved together since the 1950s, filling different data storage needs for computer users. Tape drives store and archive large amounts of data economically. Disk drives access the data with great speed.
Any data on a disk can be accessed in a few thousandths of a second. This is called random access—you don't need to search every file to find something.
A magnetic tape drive must search through all the files leading up to the one you want. This is serial access, reading one file after another.
For everyday file access, magnetic disk is faster because of its random access feature. For archiving files, tape and disk are about the same speed.
Magnetic tape is less expensive to store large amounts of data for archiving. Data centers also use disks for archiving, though to a lesser extent.
In 2009, the smallest magnetic disks are the size of a matchbox. The smallest tape drives are about the size of a small book.