Both types of disk use a magnetic recording surface to record, access, and erase data, in much the same way as magnetic tape records, plays, and erases sound or images. A read/write head, suspended over a spinning disk, is directed by the central processing unit (CPU) to the sector where the requested data is stored, or where the data is to be recorded. A hard disk uses rigid aluminum disks coated with iron oxide to store data. It has much greater storage capacity than several floppy disks (from 10 to hundreds of megabytes). While most hard disks used in microcomputers are "fixed" (built into the computer), some are removable. Minicomputer and mainframe hard disks include both fixed and removable hard disks (in modules called disk packs or disk cartridges). A floppy disk, also called a diskette, is made of plastic film covered with a magnetic coating, which is enclosed in a nonremovable plastic protective envelope. Floppy disks vary in storage capacity from 100 thousand bytes to more than two megabytes. Floppy disks are generally used in minicomputers and microcomputers.
In addition to storing more data, a hard disk can provide much faster access to storage than a floppy disk. A hard disk rotates from 2,400 to 3,600 revolutions per minute (rpm) and is constantly spinning (except in laptops, which conserve battery life by spinning the hard disk only when in use). An ultra-fast hard disk has a separate read/write head over each track on the disk, so that no time is lost in positioning the head over the desired track; accessing the desired sector takes only milliseconds, the time it takes for the disk to spin to the sector. A floppy disk does not spin until a data transfer is requested, and the rotation speed is only about 300 rpm.
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