Inside your Computer, The Hard Disk Drive
The calculations are approximates and done by me, but most of the rest of this information was shamelessly borrowed from StorageReview.com. The video is from the Colorado State College of Engineering, Dr. David G. Alciatore, PE (”Dr. Dave”) website.
To many people, a hard disk is a “black box” of sorts–it is thought of as just a small device that “somehow” stores data. There is nothing wrong with this approach of course, as long as all you care about is that it stores data. If you use your hard disk as more than just a place to “keep stuff”, then you want to know more about your hard disk. It is hard to really understand the factors that affect performance, reliability and interfacing without knowing how the drive works internally. Fortunately, most hard disks are basically the same on the inside. While the technology evolves, many of the basics are unchanged from the first PC hard disks in the early 1980s.
Every hard disk contains one or more flat disks that are used to actually hold the data in the drive. These disks are called platters (sometimes also “disks” or “discs”). They are composed of two main substances: a substrate material that forms the bulk of the platter and gives it structure and rigidity, and a magnetic media coating which actually holds the magnetic impulses that represent the data. Hard disks get their name from the rigidity of the platters used, as compared to floppy disks and other media which use flexible “platters” (actually, they aren’t usually even called platters when the material is flexible.)
The platters are “where the action is”–this is where the data itself is recorded. For this reason the quality of the platters and particularly, their media coating, is critical. The surfaces of each platter are precision machined and treated to remove any imperfections, and the hard disk itself is assembled in a clean room to reduce the chances of any dirt or contamination getting onto the platters.
The read/write heads of the hard disk are the interface between the magnetic physical media on which the data is stored and the electronic components that make up the rest of the hard disk (and the PC). The heads do the work of converting bits to magnetic pulses and storing them on the platters, and then reversing the process when the data needs to be read back.
Read/write heads are an extremely critical component in determining the overall performance of the hard disk, since they play such an important role in the storage and retrieval of data. They are usually one of the more expensive parts of the hard disk, and to enable areal densities and disk spin speeds to increase, they have had to evolve from rather humble, clumsy beginnings to being extremely advanced and complicated technology. New head technologies are often the triggering point to increasing the speed and size of modern hard disks.
In a typical consumer hard drive like what’s in your computer now, the platter spins at 7,200 RPM (85 MPH on the outside edge), and the head can seek to a track (move from one place on the platter to another) in an average of 9ms ( 9 one thousandths of a second). The SCSI hard drives that are in servers and extremely high performance desktops spin at 15,000 RPM (180 MPH on the outside edge) and can seek in an average of 3.3ms. Take a look at this video clip demonstrating the seek speed of a typical consumer hard drive.
