Marshall Brain explains that hard drives, or disks, have a hard platter that holds a magnetic medium. This medium is easily erased and rewritten, and can “remember” the magnetic flux patterns stored on it for years. The platter, usually about 5 inches in diameter, spins at about 170 miles per hour beneath an arm holding read-write heads. This arm flies at low altitude back and forth over about a 2 inch radius of the hard drive disk. When we open a spreadsheet in Excel, for example, the arm may move thousands of times in about 20 seconds, moving data between the hard drive and the RAM, where it can be manipulated quickly and easily. When we save the file, the arm records the new information to the hard drive.
“Disk drive heads literally fly over the surface of the platters at an extremely low altitude. The flight is possible because of air molecules adhering to the media surface, getting pulled under the head and causing it to lift off,” according to MicroCom Worldwide Data Recovery Services (http://www.data-master.com/BackupProtection.htm). “When all is functioning properly, the flying head technology permits theoretically indefinite use because there is no friction and hence no wear between the media surface and the head elements.” Still, eventually, often for no identifiable reason, the flying heads crash into the platter. “Data loss is the immediate and irreversible result. With head crashes, it’s not a matter of IF…it’s only a matter of WHEN.”