Reference Guide - Hard Disk Drives

Author: Charles M. Kozierok

Hard disks work by having the read/write heads fly over the surface of the disk platters. However, this floating action occurs only when the platters are spinning. When the platters are not moving, the air cushion dissipates, and the heads float down to contact the surfaces of the platters. This contact occurs when the drive spins down, as soon as the platters stop spinning fast enough, and occurs again when the spindle motor is restarted, until the platters get up to speed. Each time the heads contact the surface of the platters, there is the potential for damage. In addition to friction (and thus heat) on these sensitive components, dust can be created as the heads scrape off minute amounts of material from the platters.

Knowing that this will occur, manufacturers plan for it by incorporating special lubricants, and by ensuring that the heads normally contact the platters in a special landing zone away from user data areas. While obviously good planning, the entire way that starts and stops are handled--allowing contact with the platters--is not a great design. After several decades of handling this the same way, IBM engineers came up with a better solution.

Instead of letting the heads fall down to the surface of the disk when the disk's motor is stopped, the heads are lifted completely off the surface of the disk while the drive is still spinning, using special ramps. Only then are the disks allowed to spin down. When the power is reapplied to the spindle motor, the process is reversed: the disks spin up, and once they are going fast enough to let the heads fly without contacting the disk surface, the heads are moved off the "ramps" and back onto the surface of the platters. IBM calls this load/unload technology. In theory it should improve the reliability of the hard disk as a whole. Unfortunately, I am unaware of any other drive manufacturers using it at this time.

This feature appeared at about the same time that IBM went to the use of glass substrate platters. It may well be that the glass platters are more susceptible to damage from head contact than traditional aluminum platters. Regardless of the reason, load/unload technology is a great idea that I hope is adopted by other drive makers. This feature is also likely part of the reason why many IBM drives have very high start/stop cycle specifications. The only possible drawback I can see to this feature, other than possibly a bit of extra cost, is that spin-up time might be a bit slower on these drives.