The figure below illustrates the major components of a hard drive. We're not going to focus too much on the theory of operation of a hard drive because there are already plenty of documents available on the web that can do that. This particular article will focus solely on the "raw" hard drive and is applicable to both internal and external hard drives.
 
The Drive Controller
 
The drive controller, which isn't seen in the diagram is, for those that have seen a contemporary hard drive, the circuit card that usually takes up the better part of the bottom of the drive. The drive controller is responsible for positioning the drive heads over the drive's platters (the media used for reading and writing data,) activating the drive's heads for reading and writing data when the heads are properly positioned, interpreting and converting the data to a format the CPU can understand, and then sending the appropriate codes to back to the CPU via the I/O controller. During a write operation, the controller will send the system codes
If the drive controller is failing or non-functional, it can be repaired by replacing the controller board with a new one with the appropriate ROM, however the process may cost more than a new hard drive. If the connections are bad they should be inspected to ensure it's something simple, and not cracked traces on the controller card. Controller failures are relatively rare unless there's a manufacturing or design defect.
 
Scannerz may be able to isolate problems with the connections between the controller and the system, however if the controller itself is non-functional, for all practical purposes the drive is "turned off" and won't be seen by anything, including Scannerz.
 
The Drive Heads, Platter, and Actuator Arm
 
Data is written to and read from the drive platters via the drive's heads, which are positioned via an actuator arm. The actuator arm behaves in a manner similar to the tone arm on a record player, but it's constantly moving around on the platter at very high speeds. The drive platters are coated with a magnetically modifiable material that can be used to store information based on the localized magnetic state of a region of the platter. The information on the platters is broken down into what are called cylinders and sectors, along with tracking and positioning information. The tracking information is read by the drive heads and interpreted by the drive controller to position the drive heads to the regions of the platter that will eventually be used to read or write data on the platter.
 
During a write operation, after the controller positions the drive's heads over the appropriate region of the drive, the surface of the platter is modified by inducing an electromagnetic field over specific regions of the  platter surface. During a read operation, the spinning nature of the platter is able to induce a current into the heads much in the same way that a moving magnet can induce a current into a wire. The storage unit on the platter is the sector, which typically stores 512 bytes of information, although newer drives may have sector sizes of 4 kilobytes.
 
The most common problem with the heads, platter, and actuator arm is damage to the surface of the platter during a head crash. This may be caused by impact or by contamination of the drive chamber by foreign substances. As a drive ages it may start losing its ability to retain information. If the actuator arm jams or loses it's connections, the drive will become completely dysfunctional, possibly damaging the platter in the process. If the actuator arm is marginally functional, it may start increasingly damaging the drive platters as the heads collide with the platters.
 
A hard drive head crash will leave sections of the drive either unreadable (bad sectors,) marginally readable (weak sectors,) or both. This will often cause considerable delays as the system tries to read data from the damaged areas. Bad sectors will always generate I/O errors that are repeatable during a surface scan. Weak sectors will be readable, but will generate delays or timing irregularities that will occur consistently at the exact same locations during surface scan tests. Weak sectors can prove to be nearly as problematic as bad sectors. If the actuator arm fails or the heads start binding on the platter, the drive will become completely unusable very quickly. If the actuator arm is allowing the heads to scrape the platter surface it's typically accompanied by squealing or grinding sounds.
 
Contemporary hard drives come with a set of spare sectors that can be used to replace bad sectors if the controller detects them. If a hard drive detects a bad sector, it can be re-mapped to a spare sector if one is available, but the data contained in the original bad sectors cannot be recovered. Weak sectors may be able to have data recovered, but in some cases it may be corrupt. Weak sectors may or may not be remapped to spare sectors, depending on how severely damaged the sector is. Weak sectors may start appearing somewhat rampantly as a drive ages, especially if the drive is taken out of service, put into storage, and then put back into use after a few years.
 
A user may be able to correct problems with a drive with bad or weak sectors by reformatting the drive with an option to write data, usually binary zeros, to every sector on the drive. During the formatting and zeroing process, the drive controller will attempt to remap the bad sectors to spare sectors, as long as spares are available. If the supply of spare sectors becomes exhausted, the drive should, in our opinion, be considered unusable. Remapping bad sectors to spare sectors is a reasonable approach if only a few sectors are damaged, but if the damage is widespread we recommend replacing the drive.
 
When using Scannerz, bad and/or weak sectors are typically detected during a Normal Mode scan and confirmed during a Diagnostic Mode scan. Confirming bad or weak sectors is important because problems with bad cables or connectors can produce similar results if not confirmed.
 
The steps for performing these tests are as follows:
 
 
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1.  Perform a Normal Mode surface scan on the drive or volume
2.  If errors or significant or widespread timing irregularities exist, go to step 4.
3.  If no problems are found, it's unlikely that the problems are drive related.
4.  Put Scannerz into Diagnositics Mode
5.  Use the option to Analyze Errors and Irregularities
6.  If Scannerz identifies bad or weak sectors, testing is complete - the platters are damaged.
7.  If Scannerz doesn't detect bad sectors, this may be a cable/connector/logic board problem
 
If Scannerz doesn't detect bad sectors but is reporting errors or significant timing irregularities, check the articles on internal and external drives to ensure the problem isn't related to cables or system problems. If Scannerz detects confirmed bad or weak sectors, see the section titled Dealing with Problems. These articles are in the base article which can be viewed by clicking on the back button of your browser. It's advisable to test a problematic drive using a Phoenix Boot Volume if the problematic drive is the same drive launching Scannerz because in some cases problems with the drive may interfere with Scannerz operation and produce results that may be difficult to interpret.
 
The Spindle Motor and Actuator
 
The platter is connected to a spindle, which in turn is connected to the spindle motor. The spindle motor is not seen in diagram, but it's underneath the platter(s) and spindle. The spindle motor keeps the platter(s) rotating at a constant angular velocity, with todays drives typically using speeds between 5400 and 7200 RPM, with some higher performance drives using speeds as high as 15000 RPM. The actuator arm rotates about an actuator axis, with one end having wound coils embedded between a set of actuator magnets, known as the actuator. The position of the actuator is controlled by increasing or reducing  the current passing through these coils, which cause the actuator to move. All of these functions are controlled/regulated by the drive controller.
 
If either the spindle motor or the actuator fail, the drive won't work. It may be seen by the system but rarely will it be seen as a viable drive. Both of these are catastrophic failures that often occur suddenly. There are numerous causes, but the most common are impact to the drive or simply wear from age. A failure of this nature may be silent and fail suddenly without warning, or it may be accompanied by a variety of noises. Noises, if present, may start suddenly or come on gradually. Problems with the spindle often leave the platters of the drive intact, however it will require hard drive recovery services which can be quite expensive.
 
When using Scannerz, S.M.A.R.T. parameters are monitored including those related to some of these failures. Unfortunately, failures of this type often occur without any warning or indication. If using Scannerz and a dialog pops up indicating a S.M.A.R.T. failure occurred, you should assume the drive is in the process of failing and try to recover as much information as quickly as possible.
An Overview of a Hard Drive Problems
Overview
indicating whether or not the write operation succeeded. During a read operation, the controller will send the system blocks of data accompanied by response codes indicating the operation succeeded or failed.
 
Problems with a drive controller are usually faulty connections between the drive controller and the system or a malfunctioning on non-functioning controller. If the connections are poor or faulty, they will exhibit symptoms nearly identical to those associated with bad I/O cables for both internal and external hard drives (see the sections on internal and external hard drives for details.) If the drive controller is completely non-functional, the drive will not be seen or acknowledged by the system. If the drive controller itself is malfunctioning, the symptoms may range from the drive not being seen by the system to erratic behavior, which may in time damage the drive.