Answer to FAQ
What is meant by "media damage"?
or . . . What causes hard drive media damage?
The visible disk surface of this HD, which is part of its media, has catastrophic damage caused by a head-crash. Undamaged media areas form perfect mirrors, and the circular damaged areas show up in obvious contrast amid the reflected view in the mirror-like media surface. The image above is only slightly larger than actual size.
In a hard disk drive, "media" is the most important player of all in the long list of devices, inventions, and materials that comprise the members of this phenomenally complex orchestra we use as a data storage machine within our compu- ter. Physical damage to the media always means damage threatening to the information we've been using the drive to store for us.
Before we directly answer the above questions, for those who may not be exactly sure, let's first define what's meant by "media" in the language of hard disk drive technology. As "data" means the plural of "datum", "media" means the plural of "medium" and this stuff is the physical material characterized by a primary enabling attribute or ability, i.e., its magnetic property, to hold or contain the information stored on the disk drive by the user of the computer (or other system) to which the drive is connected.
Media consist of very thin, extremely smooth, super-delicate coatings of magnetic material which have been deposited, during the platter manufacturing process, upon both sides of each disk drive platter or substrate. This coating, also called plating, is so delicate that if any physical contact whatsoever occurs with it, it's critical magnetic property is altered, resulting in functional degradation at the particular point of contact. If user data has already been recorded at the same spot where contact occurs, that data is subject to permanent loss. This is a good place to point out that whenever hard disk drives fail, media damage or corruption is very much likely to be closely involved with the failure event.
So, in answer to the above two-pronged FAQ, for the first part: "What is meant by media damage?" we now shall define "damage" as the condition of the media where any physical change has occurred (other than magnetic recording), causing alteration to the finely tuned and balanced properties of the coating on any and every disk surface. Physical damage is inflicted upon the media (or recording "medium" to use the singular) any time the hard drive is powered on, the disk platters are spinning and any contact takes place between this surface and the hard drive's R/W (read/write) heads. The R/W heads normally fly over the surface on an air-bearing. "Flying" means "like an airplane" — there is supposed to be nothing but air molecules between the heads and the surface of the disks. Hard drives are designed in such a way that it's very unlikely that anything other than one of the heads could make such contact, since both heads and disk platters are sealed inside a controlled environment created within the drive's interior. NOTE: Additional information about media and the air-bearing enabled "flying head" dynamic, employed in the design of all computer hard disk drives, may be viewed on our FAQ page about head crashes.
Here is another pictorial example of media damage. Instead of the two concentric rings of disk platter scoring shown in the first picture, the degradation of the media in this second image is evident by the visible haze-like damage to the originally mirror-like surface. This indicates the entire surface has been corrupted by physical contact with its respective head, and took place after the head ceased flying, and before the R/W heads ceased seeking to all areas. Sometimes the disks continue to spin indefinitely, and the magnetic material gets scraped off the platters entirely by the crashed heads, as pictured and discussed elsewhere on this site.
So, in summation of our definition for media damage, the data we store and later retrieve from a hard drive is recorded magnetically within a plating that has been coated onto the surface of each disk platter. The plating is extremely delicate, and its performance relies upon remaining physically isolated from all manner of other physical objects, such as particles floating in unfiltered air, and including the R/W heads. A hard disk drive head must fly at an altitude about two-millionths of an inch away from the platter surface in order for it be able to make recordings of data to be stored (writing), and then later play them back (reading). The two-way, I/O communication between the drive's heads and the media, carried out by means of the process we've described above, involves literal transmission and detection of minute magnetic force fields across this miniscule, miroscopic, two micro-inch gap.
Two Types of Damage
While there are two types or categories of media corruption, there's but one most important factor that applies to both. Both types of media damage have one salient attribute in common and this fact has definitive impact on all hard drive users. Either type may occur anywhere on the media and thus affect virtually any part of the drive's media in a random way, at a random physical location, completely without regard or relation to any logical information (read "user data"). In other words, there is no connection between the value that we, the hard drive's user, place upon having access to our data and whether or not the physical location of media damage on our drive will coincide and obliterate that data. It really just boils down to the luck of random events and is therefore entirely unpredictable. If media damage can be prevented from becoming too wide-spread, in the majority of cases the damage of which we speak typically does not coincide and collide with our most critically important files, and our data can be safely recovered.
The two different, distinct types of physical change resulting in media corruption are:
- (1) instantaneous damage, and
- (2) gradual damage accumulating during routine usage or aging process.
Instantaneous damage occurs when any one of the R/W heads in a hard drive touches down or makes any physical contact with the platter surface over which the head flies within areas where data can be recorded (as a cost-savings design, some drives, especially those built 1983-2000, "park" heads in a zone nearest the center, making the only case of exception). Instantaneous damage comprises media corruption taking place either as a causitive event precipitating drive failure or as a concommitant result of failure. For example, a hard drive, during normal use, can develop damage on parts of its media that contains recorded operational code supporting its every function (e.g., firmware) and the drive will fail. Or, the drive can fail by means of a lightening-strike to the local power grid that causes an electrical spike to pulse through all of the drive's electronics, including the R/W heads, which in turn triggers a head crash whereby immediate deformation and devastation of media surface is produced at the crash site.
Gradual damage is typically an entirely unnoticeable process to we users of hard drives. One of the most common forms of this phenomenon is known by disk drive design engineers as grown defects. It may be thought of as "wear & tear", but the expression is placed in quotation marks because, strictly speaking the process most people envision as "wearing out", is not an accurate description of what's really going on with the magnetic recording medium over elapsed time. In the absence of physical contact, the degrading process in reality is more akin to a form of decay. All this is pretty much an esoteric, academic issue, so for the everyday user to think of it as though it were "wear and tear" is for all practical purposes, a satisfactory analogy.
With thin-film magnetic data recording medium produced for use in disk drives built since the turn of the century, there is no such thing as perfect, defect-free media. Sophisticated, highly evolved defect management schemes are now required. In order to realize a storage capacity of more than half a trillion bytes on a single three and a half inch diameter disk (as typical circa 2010), recorded information in today's hard drives is more tightly and densely recorded than ever, packed to a degree even scientists could scarcely imagine less than ten years ago. To facilitate this extremely high areal density (as it's called), all hard disk drive designs incorporate, among their thousands of other design parameters, means to handle numerous defects, defects that are inevitably found in every single platter manufactured.
Not only does every brand new hard drive come out of the box with thousands of media defects (invisible to the user), but by means of prolonged, repeated heating-cooling cycles encountered in normal operation, the media undergoes aging, and even more media defects emerge or "grow" (in number) within the media comprising the data recording areas on a hard drive. As defect management maximizes the capacity of data that can be recorded upon each platter, and at the same time increases the cost-effectivity of platter manufacture (by reducing the number of quality control rejections), all drives are designed to deal with new, emerging defects that arise after manuafacturing is complete and its lifetime of usage has begun.
After producing a "defect map" during one of the drive's numerous manufacturing processes, data being recorded is prevented from being written or stored at the location of any defect. Once the drive is in actual use, a similar automatic function enables the drive to automatically prevent subsequent data recordings from being made in any location where new defects have appeared, by adding new locations as they emerge to the grown defects map. All of this is done in a way that is transparent even to the host system, and so of course the user is absolutely none the wiser. When it comes to data storage, that's the way we users like it.
And, here's the rub. Obviously, everything has limits, and naturally a disk drive has a limited, finite amount of storage space to hold the information in its grown defect map. So what happens if more defects accumulate than the drive has capacity to log? You guessed it, the drive fails. In situations where instantaneous damage is taking place in such a manner that it's not too rapid and catastrophic, defects arising can be handled by this same design feature, and the drive can operate in a seemingly normal manner for a time (weeks or days). However, once media defects begin accruing as a result of frictional physical contact with a R/W head, their number increases many orders of magnatude faster than they would by means of any normal media aging process, the finite capacity of the grown defect map is overwhelmed in fairly short order, and the drive fails.
Data Recovery with Media Damage
Damages sustained upon the media within a hard disk must be carefully diagnosed to evaluate the likely outcome if recovery services are performed on the drive. In cases where important data was recorded in a physical location coincident with media damage, the nonrecoverablity in such cases must be reported to a prospective client to inform any decision to be made about how to proceed. MicroCom provides a full diagnostic evaluation report (DER) to each client, in every case, prior to performing any data recovery service for charge.
Narrative text written and photos by
S.E. Fowler / Steve Fowler / Stephen E. Fowler, © 2010
All rights reserved.
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