Abstract:
An information storage system has structure with a head that can effect information transfers with respect to a storage medium. According to one feature, a control portion monitors a characteristic of information read from the storage medium, and can take action to reduce the likelihood of non-recoverable errors based on the characteristic. According to a different feature, the control portion moves the head to a position adjacent a portion of a surface on the storage medium, waits while moving the head relative to the surface, and then moves the head to another position adjacent a different portion of the surface. According to another feature, the control portion calculates a distance of the head from the surface based on information read from the surface, and determines based on this distance whether the head or the storage medium may fail to satisfy a criteria.

Description:
This application claims the priority under 35 U.S.C. §119 of provisional application No. 60/439,892 filed Jan. 13, 2003. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     This invention relates in general to information storage systems and, more particularly, to techniques for reducing or avoiding the incidence of errors in data stored in and retrieved from information storage systems. 
     BACKGROUND OF THE INVENTION 
     Over the past twenty years, computer technology has evolved very rapidly. One aspect of this evolution has been a progressively growing demand for increased storage capacity in memory devices. In order to provide a high storage density at a reasonable cost, one of the most enduring techniques has been to provide a rotatable hard disk with a layer of magnetic material thereon, and a read/write head which is supported for movement adjacent the disk and can transfer information to and from the disk. 
     In an arrangement of this type, if airborne dust, smoke, vapors or other contaminants are present, they can be attracted to the disk or the head, for example by any electrostatic charge that may be present. Because the head is disposed closely adjacent the disk during normal operation, contaminants on the disk can be transferred to and progressively build up on the head. Eventually, the buildup of contaminants will be sufficient to interfere with the interaction between the head and disk, thereby increasing the error rate until the device will not operate. Also, if a large particle becomes trapped between the head and disk, it can cause damage to the magnetic surface present on the disk. Alternatively, such a trapped particle can cause stored information to be erased, without physical damage to the disk. In this regard, the particle can rub on the disk with sufficient pressure to elevate the temperature of the magnetic coating until magnetic information is thermally disorganized, or in other words erased. 
     In order to avoid these problems due to contaminants, most hard disk drives have the disk and head disposed within a sealed enclosure, so that the disk and head are not exposed to whatever airborne contaminants may happen to be present externally of the enclosure. This approach works well where the entire hard disk drive device is permanently installed in a computer. In other types of systems, however, a hard disk is provided within a removable cartridge, and it is desirable that the cartridge not include the read/write head. 
     In this regard, there are advantages to placing a read/write head and its support structure within the drive which receives the cartridge, rather than in the cartridge. For example, a typical user will have several removable cartridges for each drive. Thus, in terms of overall system cost, it is cheaper to provide one head with its support in the drive, rather than to provide several heads with supports which are each disposed in a respective one of many cartridges used with the drive. However, in removable cartridges, there is a problem in regard to keeping the head clean. 
     More specifically, in order to permit the head from the drive to access the disk within the cartridge, the cartridge is not provided with a sealed enclosure of the type discussed above. Instead, the cartridge is provided with an opening through which the head of the drive can be inserted into the cartridge. In some cases, a movable shutter is provided to obstruct the opening when the cartridge is not disposed in the drive. However, when the cartridge is disposed in the drive, the shutter moves to an open position. Thus, regardless of whether or not a shutter is present, when the cartridge is in the drive, there is an opening which gives the head access to the interior of the cartridge. That opening also necessarily gives ambient air access to the interior of the cartridge, along with any dust, smoke, vapor or other contaminants that are carried by the ambient air. 
     In order to reduce the incidence of errors, pre-existing systems of this type have typically taken the approach of using a relatively low density for the data stored on the magnetic disk. While this approach has been generally adequate for its intended purposes, it has not been completely satisfactory in all respects. In particular, and as discussed above, there is a progressively increasing demand for progressively higher storage capacities in devices of this type. To achieve this, there is a need to use higher data storage densities, which in turn presents an increased likelihood of errors in data stored on and retrieved from the magnetic disk. 
     SUMMARY OF THE INVENTION 
     From the foregoing, it may be appreciated that a need has arisen for improved techniques for minimizing the effects of airborne contaminants in information storage systems. One form of the invention relates to an information storage system which includes an information storage medium and structure operable to effect information transfers with respect to the information storage medium. This form of the invention involves: monitoring a characteristic of information read by the structure from the storage medium, including determining whether the characteristic satisfies a predetermined criteria; and responding to a determination that the characteristic fails to satisfy the predetermined criteria by carrying out a course of action which includes a selected action that reduces the likelihood of non-recoverable errors in data read by the structure from the storage medium. 
     A different form of the invention relates to an information storage system which includes an information storage medium having an information storage surface with first and second portions, and which includes structure operable to effect information transfers with respect to the first portion of the surface, the structure including a head which is movable relative to the surface. This form of the invention involves: moving the head from a first position spaced from the surface to a second position in which the head is adjacent the second portion of the surface; waiting a predetermined time interval while effecting relative movement of the head and the surface with the head adjacent the second portion of the surface; and thereafter moving the head to a third position in which the head is adjacent the first portion of the surface. 
     Yet another form of the invention relates to an information storage system which includes an information storage medium having an information storage surface, and structure operable to effect information transfers with respect to the surface, the structure including a head which is movable relative to the surface. This form of the invention involves: calculating a distance of the head from the surface based on information read by the structure from the surface; and determining as a function of the distance whether one of the head and the storage medium is likely to fail to satisfy a predetermined operational criteria. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A better understanding of the present invention will be realized from the detailed description which follows, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a diagrammatic view of an apparatus which is an information storage system, and which embodies aspects of the present invention; 
         FIG. 2  is a flowchart showing a portion of a program which is executed by a processor in the system of  FIG. 1 ; 
         FIG. 3  is a graph showing hard read errors in relation to a soft error rate for the system of  FIG. 1 ; and 
         FIG. 4  is a flowchart showing a procedure used by a control circuit in the system of  FIG. 1  in order to differentiate between different types of problems. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a diagrammatic view of an apparatus which is an information storage system  10 , and which embodies aspects of the present invention. The system  10  includes a drive  12  having a recess  13  into which an information storage cartridge  14  can be removably inserted. The cartridge  14  includes a housing  18 , and on one side of the housing  18  there is an opening, which is not separately illustrated. In  FIG. 1 , the opening is on the side of the housing  18  nearest the bottom of  FIG. 1 . 
     The cartridge  14  has, within the housing  18 , a hard disk  21  which is fixedly mounted on a rotable spindle  22 . The hard disk  21  includes a substrate, for example made of metal, glass or ceramic. The side of the disk  21  which is visible in  FIG. 1  is coated with a magnetic material of a known type. For simplicity and convenience in explaining the present invention, the disk  21  of  FIG. 1  is assumed to have this magnetic coating on only one side thereof. However, it will be recognized that the disk  21  could have the magnetic coating on each side, or that there could be several disks  21  all supported on the spindle  22 , and each having a magnetic coating on one or both sides. 
     The magnetic coating on the disk  21  has a radially outer portion  26 , which is a circular loading/unloading track. This track  26  is not used for operational data storage, but has servo information of a known type stored along it. 
     Near the spindle  22 , the radially inner portion of the magnetic coating has a further circular portion  28 , which is referred to as a reserved track or debug track. The track  28  is not used to store operational user data, but has servo information stored along it. The drive  12  can use the track  28  to store test data and to then read back the stored test data. The portion of the magnetic coating between the tracks  26  and  28  is a region  31  which includes a number of circular tracks that are not separately illustrated. The region  31  stores servo information of a known type for a number of different tracks, and is also used for operational storage of user data. 
     The drive  12  includes a motor  51 . As indicated diagrammatically by a broken line  52 , the motor  51  is operationally coupled in a known manner to the spindle  22  in the cartridge  14  when the cartridge  14  is removably disposed in the drive  12 . The motor  51  effects rotation of the spindle  22 , and thus rotation of the disk  21 . 
     The drive  12  also includes an actuator  56  of a known type, such as a voice coil motor. The actuator  56  has a pivot axle  58 . An actuator arm  57  has one end fixedly secured to the pivot axle  58 , and the actuator  56  can pivot the pivot axle  58  so as to effect movement of the arm  57  in directions indicated diagrammatically by a double-headed arrow  59 . At the outer end of the arm  57  is a suspension  61  of a known type, which supports a magnetic head  62  of a known type. The term “head” is used here to collectively refer to two known elements, which are sometimes referred to separately as the slider and the read/write head. An outwardly projecting tab  63  is provided at the outer end of the suspension  61 , for a purpose described later. 
     The drive  12  has a support leg  71 . At its outer end, the leg  71  supports a ramp  72  and a cleaning pad  73 . The ramp  72  has a detent  76  at a location spaced inwardly from each end, for a purpose described later. The cleaning pad  73  is made from a material of a known type, and has a textured surface on the side thereof which is visible in  FIG. 1 . 
     The actuator  56  can pivot the arm  57  to a position in which the tab  63  is engaging the detent  76  of the ramp  72 . In this position, the head  62  is offset from the cleaning pad  73 . This is known as the park position of the head  62  and arm  57 . The arm  57  and head  62  will be in this park position when the cartridge  14  is not present in the drive  12 , and thus will also be in this position as the cartridge  14  is being manually inserted into the recess  13  in the drive  12 . As the cartridge  14  is being inserted into the drive  12 , the arm  57  and leg  71  move into the housing  18  of the cartridge, through the above-mentioned opening which is provided in one side of the housing  18 . When the cartridge  14  has been fully inserted, the actuator  56  can move the arm  57  counterclockwise in  FIG. 1 , so that the tab  63  leaves the detent  76  and slides down the portion of the ramp  72  which is to the left of the detent  76 , until the head  62  is adjacent the magnetic coating on the surface of the disk  21 . As the head  62  nears the disk  21 , the head  62  will be aligned with the loading/unloading track  26 . 
     As the head  62  nears the disk  21 , the motor  51  will be rotating the disk  21  at a normal operational speed. During normal operation, the head  62  does not actually engage the disk  21 . Instead, in a known manner, the rotation of the disk  21  induces the formation of an air cushion between the disk surface and the head  62 . This air cushion is commonly known as an air bearing. The head  62  floats on the air bearing while reading and writing information to and from the disk, without direct physical contact with the disk. The spacing between the head and disk is commonly referred to as the “fly height” of the head, and in the disclosed embodiment is about 0.01 to 0.015 μm. As the arm  57  is pivoted by the actuator  56 , the head  62  moves approximately radially with respect to the rotating disk  21 , within a range bounded by a location aligned with the loading/unloading track  26 , and a location aligned with the reserved track  28 . 
     The drive  12  includes a channel circuit  101 , which is a circuit of a known type. The channel circuit  101  receives at  102  an output signal from the head  62 . Based on signals received from the head  62 , the channel circuit  101  outputs various different types of information. This information includes a position error signal PES of a known type, as well as data (DATA) which has been read from the disk  21 . The channel circuit  101  also outputs information in the form of a known parameter PW50, which represents the pulse width at 50% amplitude of the servo pulses read from the disk  21  by the head  62 . The channel circuit  101  also outputs a channel quality parameter CQP, which is a further parameter of a known type. In particular, the channel circuit  101  takes an analog signal representing the data read from the disk by the head  62 , samples the signal, and compares the sampled information to an idealized reference. The difference is used to calculate the channel quality parameter CQP in a known manner. 
     The drive  12  includes a control circuit  110 , which receives information from the channel circuit  101 , including the position error signal PES, the parameter PW50, the channel quality parameter CQP, and the data information DATA. The control circuit  110  can also receive from the channel circuit  101  some other information which is known in the art but not specifically depicted in  FIG. 1 , such as automatic gain control (AGC) information for both servo information and data information read from the disk, and information relating to operation of a finite impulse response (FIR) filter in the channel circuit  101 . 
     The control circuit  110  has an output  116  which is coupled to and controls the motor  51 , and a further output  117  which is coupled to and controls the actuator  56 . The control circuit  110  can be coupled through a host interface  118  to a not-illustrated host computer of a known type. The control circuit  110  includes a processor  111 , a read only memory (ROM)  112  that stores static data and a program executed by the processor  111 , and a random access memory (RAM)  113  that is used by the processor  111  during program execution to store dynamically changing data. 
     In any hard disk system, airborne contaminants such as dust or smoke particles can create problems in regard to interaction between the head and the disk. For example, contaminants can build up on the head until they degrade the operation of the head. Also, a medium to large particle (larger than about 0.5 μm) could become lodged between the head and the disk, and then scratch the magnetic coating on the surface of the disk. Therefore, in order to achieve high data storage densities, there is a need to address this problem of airborne contaminants. For many years, the traditional approach was to provide a sealed housing which contained the disk, the actuator, the actuator arm, and the head, so that these components were all completely isolated from airborne particles in the environment external to the housing. However, in the case of a removable data storage cartridge, each cartridge would need to include not only a hard disk, but also an actuator and a magnetic read/write head. Providing these components in each cartridge would significantly increase the cost of each cartridge. Consequently, in the case of a removable data storage cartridge, it is desirable for the actuator and the head to be a part of the drive, and to be provided with access to the hard disk through an opening in the housing of the cartridge. One example of such a system is an information storage system that has been commercially available for several years under the trademark JAZ from Iomega Corporation of Roy, Utah, which is the assignee of the present invention. 
     In systems of this latter type, the opening in the housing of the cartridge exposes the disk and the magnetic head to airborne contaminants. Consequently, in order to minimize the adverse effects of these airborne contaminants while also providing reliable data storage, it has been necessary in pre-existing systems to keep data storage densities relatively low. For example, pre-existing hard disk systems with a sealed housing often have storage capacities of 20 GB or more, whereas a pre-existing system with an unsealed housing would typically have a storage capacity of only about 1 GB to 2 GB, due to the lower storage densities. 
     However, there is a progressively increasing demand for greater storage capacities in removable cartridges, as well as an increasing demand for progressively lower prices for the cartridges. Consequently, the present invention relates to techniques that can help minimize the effects of airborne contaminants. In the case of a cartridge having a hard disk in an unsealed housing, storage densities can be achieved that are significantly higher than in pre-existing systems with unsealed housings. 
     In this regard,  FIG. 2  is a flowchart showing a portion of a program which is stored in the ROM  112  of  FIG. 1 , and which is executed by the processor  111 . As mentioned above, when the cartridge  14  of  FIG. 1  is being inserted into the drive  12 , the tab  63  on the support arm  57  is disposed in the detent  76  of the ramp  72 , which is the park position of the arm  57  and head  62 . The initial portion of the flowchart of  FIG. 2  represents a technique for moving the head  62  from its park position to an operational system adjacent the disk  21 . 
     More specifically, in block  150  the control circuit  110  sets a count to zero. Then, in block  151 , the control circuit  110  causes the actuator  56  to pivot the arm  57  so that the tab  63  leaves the detent  76  and slides down the left side of the ramp  72 , causing the head  62  to approach the loading/unloading track  26  while an air bearing forms between the head  62  and the disk  21 . It is possible that, while the head  62  was in the park position, a medium to large airborne particle (larger than about 0.5 μm) may have settled on the head  62 , or on the disk  21  at a location along the loading/unloading track  26 . As the head  62  is being moved into a position closely adjacent the loading/unloading track  26 , such a particle could become trapped between the head  62  and disk  21 , and could immediately start to scratch the magnetic coating on the disk. Alternatively, such a trapped particle could cause stored information to be erased, without physical damage to the disk. In this regard, the trapped particle could rub on the disk with sufficient pressure to elevate the temperature of the magnetic coating until magnetic information is thermally disorganized, or in other words erased. 
     In the embodiment of  FIG. 1 , such a trapped particle could produce some scratching of the magnetic coating in the region of the track  26 , or thermal disorganization of the magnetic coating there, but this will not be catastrophic, because user data is not stored there. However, it is desirable to attempt to dispose of any such particle before the head  62  and the trapped particle are moved away from the track  26  to the region  31  where user data actually is stored. 
     Therefore, after the head  62  has been moved to a position adjacent the track  26  in block  151 , the control circuit  110  proceeds to block  152 , where it waits for approximately 2 to 10 revolutions of the disk  21 . Then, at block  153 , the control circuit  110  checks for any abnormality in a selected evaluating parameter, such as the position error signal PES. It would alternatively be possible at this point to check for abnormalities in each of two or more parameters, such as two or more of the position error signal (PES), the servo automatic gain control (AGC) signal, information from a finite impulse response (FIR) filter, or any other suitable parameter. However, for clarity and simplicity, the following discussion assumes that only a single evaluating parameter is used, which is the PES. 
     At block  156 , if the evaluating parameter is found to be normal, it means that there is probably no trapped particle between the head  62  and disk  21 , and block  157  is skipped. On the other hand, if the evaluating parameter is found to be abnormal, then a particle may be trapped between the head  62  and disk  21 , and the control circuit  110  proceeds to block  158 . In block  158 , the control circuit  110  checks to see if the count is still zero, or in other words whether this is the first pass through block  156 . If so, then control proceeds to block  159 , where the control circuit  110  increments the count. Then, in block  157 , the control circuit  110  takes some appropriate corrective action. 
     In particular, in block  157 , the control circuit  110  causes the actuator  56  to pivot the arm  57  clockwise in  FIG. 1 , so that the tab  63  slides back up the ramp  72 , to and beyond the position of the detent  76 , until the head  62  is engaging the cleaning pad  73 . To the extent that any particle may have become trapped between the head  62  and disk  21 , the particle will typically be carried by the head  62  to the cleaning pad  73 , where it will be removed by the cleaning pad. After the head  62  has been cleaned by the cleaning pad  73 , it will promptly be returned to a position adjacent the disk  21  before any further particle can settle along the loading/unloading track  26 . In particular, the control circuit  110  will promptly cause the actuator  56  to pivot the arm  57  counterclockwise in  FIG. 1 , so that the tab  63  slides back along the ramp  72  until the head  62  is again adjacent the track  26 , with an air bearing formed therebetween. Once a proper fly height has been established between the head and disk, the geometry of the head is such that it would be very rare for a medium to large particle to become trapped between the head and disk. 
     Control then returns to block  152  to check again for the existence of a problem. When control again reaches block  156 , if it is determined that there is still a problem, then control proceeds to block  158 , where it will be determined that the count is now greater than zero. Consequently, the control circuit  110  will proceed to block  163 , where it moves the arm  57  and head  62  to the park position, in which the tab  63  engages the detent  76 . The control circuit  110  then transmits a notice of an error condition through the host interface  118  to the not-illustrated host computer. The control circuit  110  then ceases normal operation, in an attempt to halt operation of the system  10  before any non-recoverable hard error occurs. 
     Referring again to block  156 , if it is determined that the evaluating parameter is normal, then control proceeds to block  161 . If the evaluating parameter is determined to be normal during the first pass through the loop, then it means there was no problem. On the other hand, if the evaluating parameter is determined to be normal during the second pass through the loop, then it means that there was a problem but that the cleaning operation carried out in block  157  resolved it. 
     Referring now in more detail to block  161 , the control circuit  110  of the drive  12  carries out a further operation which is intended to compensate for airborne contaminants. In this regard, and as mentioned above, airborne contaminants can build up on the head  62  over time, causing problems such as read errors, or instability in the spacing between the head and disk. 
     There are two types of read errors. One type is known as a recoverable or soft error, due to the fact that error correction techniques of a known type can be used to correct the error. The other type of error is known as an unrecoverable or hard error, which is an error that cannot be corrected, even through the use of error correction techniques. The frequency of occurrence of soft errors is the soft error rate (SER), which can also be referred to as the soft bit error rate (SBER). The frequency of occurrence of hard errors is the hard error rate (HER), which can also be referred to as the hard bit error rate (HBER). 
       FIG. 3  is a graph presenting empirical data for the system of  FIG. 1 , showing the number of hard read errors in relation to the soft error rate in units of bits per error. (This is the reciprocal of the units used by others in the industry, which is errors per bit). It will be noted that the number of hard read errors increases exponentially as the soft error rate approaches  104 . According to a feature of the present invention, the soft error rate (SER) of the system  10  of  FIG. 1  is constantly monitored by the control circuit  110 , and an error condition is generated if the SER approaches a predetermined threshold value, which is 10 4 . 
     More specifically, with reference to block  161  in  FIG. 2 , the control circuit  110  causes the actuator  56  to move the arm  57  until the head  62  is aligned with the reserved track  28  at the radially inner portion of the disk  21 . One reason that the track  28  is located near the radially inner portion of the disk  21  is that, in a typical zone recording scheme, the track  28  has the highest data storage density, and thus will tend to have the highest susceptibility to errors. When the head  62  is aligned with the track  28 , the control circuit  110  uses the head  62  to write test data to the track  28 , to read back this test data, and to then check this test data for soft errors, in order to calculate a soft error rate SER. Then, at block  162 , the control circuit  110  checks to see if the SER is below a predetermined threshold, which in the disclosed embodiment is 10 4 . If the SER is found to be below the threshold, then the control circuit  110  proceeds to block  163 , where as discussed above it moves the arm  57  and head  62  to the park position, in which the tab  63  engages the detent  76 . The control circuit  110  then transmits a notice of an error condition through the host interface  118  to the not-illustrated host computer. The control circuit  110  then ceases normal operation, in an attempt to halt operation of the system  10  before any non-recoverable hard error occurs. 
     Although the disclosed embodiment checks the soft error rate SER at block  161 , it would alternatively be possible for the control circuit  110  to check any of a number of other parameters. As one example, the control circuit could alternatively check the position error signal (PES). Consequently, the discussion here of checking the SER is merely one example of a suitable parameter. 
     Referring again to block  162 , if the control circuit  110  determines that the calculated soft error rate SER is not below the selected threshold, then it proceeds to block  164 . In block  164 , the control circuit  110  moves the head  62  to the data region  31  between the tracks  26  and  28 , and proceeds with normal operation involving reading and/or writing of operational user data. 
     As discussed above, blocks  153  and  156  in  FIG. 2  are used to determine whether to execute or skip the block  157 . As an alternative, blocks  153  and  156  could be omitted from the flowchart of  FIG. 2 , such that control would proceed from block  152  directly to block  157 . In other words, the head  62  would be moved into operational alignment with the loading/unloading track  26 , the system would wait for about 2 to 10 revolutions of the disk  21 , and then the head would be unconditionally moved to the cleaning position in which it engages the cleaning pad  73 . 
     With reference to block  161 , the flowchart of  FIG. 2  shows that the soft error rate SER is promptly checked each time the head  62  is moved into operational alignment with the disk after spending a period of time in the park position. However, the soft error rate SER can be checked at other times during system operation, either in addition to or in lieu of the check shown in block  161 . For example, the SER could be checked just before the head is unloaded from the disk, or could be checked using operational user data, as that user data is being read from the data region  31  of the disk  21  in response to a user request. As still another alternative, a check of the SER could be carried out during an idle condition of the system  10 . For example, when the head  62  is in its park position, the control circuit  110  could briefly move the head away from the park position in order to write and read test data to and from the reserved track  28 , and could then calculate the SER based on this test data. 
     In  FIG. 2 , if the detected soft error rate SER is found to be below the threshold at block  162 , the control circuit  110  essentially shuts down system operation at block  163 . Alternatively, however, the control circuit  110  could take some other course of action. For example, the control circuit  110  could (1) prevent a user from writing data, (2) allow a user to write data only in a write-with-verify mode, or (3) initiate recovery activities such as head cleaning in order to improve the error rate performance. 
     In blocks  161 - 163  of  FIG. 2 , an error condition is generated after checking the soft error rate SER at just one radius of the disk, which is the radius associated with track  28 . Alternatively, however, when the SER is found to be below the threshold at any radius, the SER could be checked again at a different radius, and then the error condition would be generated only if the SER was found to be below the threshold at each such radius. 
     A further consideration relates to the fact that the blocks  161  and  162  involve evaluation of the soft error rate SER. It would alternatively be possible to check for a degradation of system operation using some parameter other than the SER, such as the channel quality parameter CQP which was discussed above in association with  FIG. 1 . 
     As discussed above, airborne contaminants can build up on the head  62  in a manner which could degrade system operation and cause a loss of user data. On the other hand, degradation of system operation could also occur as a result of other factors, such as damage to the magnetic coating on the hard disk. In the past, it has been difficult to differentiate between problems which are due to degradation of the head and problems which are due to other factors, such as media damage.  FIG. 4  is a flowchart showing a procedure used by the control circuit  110  of  FIG. 1  to differentiate between these various types of problems. 
     More specifically, and with reference to  FIG. 4 , the procedure begins at block  201 , where the control circuit  110  checks the fly height of the head  62  while the head is reading user data from the data region  31  of the disk. In the disclosed embodiment, the fly height is determined by checking a parameter received from the channel circuit  101 . There are a variety of different parameters which could be used as the evaluating parameter. For example, the evaluating parameter could be the channel quality parameter (CQP), automatic gain control (AGC) information for data or servo information read from the disk, information relating to what the finite impulse response (FIR) filter in the channel circuit  101  is currently doing to attempt to equalize pulse shapes read from the data sectors, soft error rate (SER) information, or the PW50 information. Persons skilled in the art will recognize that, as to some of these parameters, the evaluation would involve a comparison of the current value to some form of reference value, which could be either a predetermined value, or a prior measured value of the parameter which was saved at some previous point in time. As to the PW50 information, an estimate of PW50 can be calculated using known information about the drive, and using information obtained from the FIR filter which is commonly referred to as tap values. In this regard, the FIR information applied to a gray code in a servo field would yield PW50 for the servo field. A similar approach can be used to obtain PW50 information for a data field. 
     After determining the fly height at block  201 , the control circuit  110  proceeds to block  202 , where it evaluates whether this fly height is degraded, or in other words whether the fly height falls outside what is considered a normal operational range for the fly height. If no problem is detected, then the control circuit  110  exits the procedure of  FIG. 4  at block  203 , representing a determination that the system does not appear to have a significant problem. 
     On the other hand, if it is determined at block  202  that the fly height is degraded, then the control circuit  110  is faced with the possible existence of a problem, and takes further action to investigate. In particular, at block  206 , the control circuit  110  checks the fly height while the head is reading servo information from the data region  31  of the disk. At block  207 , the control circuit  110  evaluates whether this particular fly height is degraded from a normal condition. If not, then the control circuit  110  proceeds to block  208 , where it takes the data which was checked at  201 , rewrites this data to the disk, and then determines the fly height again while it is reading back this rewritten data. At block  211 , the control circuit  110  checks to see whether the fly height determined from the rewritten data is degraded. If not, then the control circuit  110  exits the procedure at  212 , representing a determination that the problem has been resolved. 
     On the other hand, if it is determined at block  211  that the fly height for the rewritten data is degraded, then the control circuit  110  assumes that the disk has a localized defect, and at block  213  it moves the data to some other location on the disk. The control circuit  110  then exits the procedure of  FIG. 4  at block  216 , representing a determination that the disk has localized media damage, and that the problem has been resolved by moving the data to a different part of the disk, and by mapping the damaged part of the disk out of the definition which identifies portions of the disk that are available to a user. 
     Looking again at block  207 , if the control circuit  110  determines that the fly height for the servo information is degraded, it proceeds to block  217  to carry out further investigation. In block  217 , the control circuit  110  checks the fly height for servo information at a radius on the disk which is different from the radius used in block  206 . Then, at block  218 , if the fly height for the further servo information is found to be acceptable, the control circuit  110  concludes that the portion of the disk used in blocks  201  and  206  has localized media damage, and proceeds to block  213  in order to move the data from that portion of the disk to a different portion of the disk, in the manner discussed above. 
     On the other hand, if the control circuit  110  determines at block  218  that the fly height associated with the further servo information is abnormal, then it proceeds to block  221 , where it sets a count to zero. Then, in block  222 , it moves the head  62  to the reserved track  28 , writes test data to the reserved track, and then reads back that test data in order to check the fly height at the reserved track  28 . If the fly height for the reserved track  28  is found to be acceptable at block  223 , then the control circuit  110  proceeds to block  224 , where it checks to see if the count is greater than zero. If so, then the control circuit  110  exits the procedure of  FIG. 4  at block  226  while generating an error condition which indicates that the disk is bad and should be replaced. In response to this error indication, a user might transfer all of the user data in the cartridge  14  to a new and identical cartridge, and then discard the problematic cartridge  14 . On the other hand, if the count is found to be greater than zero in block  224 , then control proceeds to block  225 , which is discussed in more detail below. 
     Referring again to block  223 , if the control circuit  110  determines that the fly height is degraded as to the reserved track  28 , then at block  227  it increments the count, and proceeds to block  228 . At block  228  the control circuit  110  causes the actuator  56  to move the head  62  until it engages the cleaning pad  73 , and to then move the head  62  back to the reserved track  28 . Then, at block  231 , the control circuit  110  checks to see whether the count is greater than 1, or in other words whether blocks  222 - 223  and  227 - 228  have already been executed twice. If they have been executed only once, then the control circuit  110  returns to block  222 , in order to execute these blocks again. This time, if the fly height is found to be acceptable at block  223 , it means the cleaning operation was successful. Therefore, since the count will now be found to be greater than zero in block  224 , the control circuit will exit the procedure of  FIG. 4B  at block  225 , representing a determination that the problem has been resolved. 
     Referring again to block  231 , if it is determined that the count is greater than one, or in other words that there is a problem which was not resolved by a cleaning operation at block  228 , then the control circuit  110  proceeds to block  232 . In block  232 , the control circuit  110  generates a message through the host interface  118 , which causes the host computer to ask the user to replace the cartridge  14  with a different cartridge. The control circuit  110  waits for the insertion of the replacement cartridge, and then proceeds to block  233 , where it checks the fly height for the new disk. Then, at block  236 , it evaluates whether the fly height determined for the disk in the new cartridge is acceptable. If the fly height is acceptable, then the control circuit  110  exits the procedure of  FIG. 4  at block  237 , representing a determination that the problem was in the original cartridge  14  rather than in the drive  12 , and has been resolved. Alternatively, if it is found at block  236  that the fly height is not acceptable, then the control circuit  110  exits the procedure of  FIG. 4  at block  238 , while notifying the host computer that the drive  12  is problematic and should be replaced. The problem with the drive  12  may, for example, be due to degradation of the operation of the head  62  in the drive  12 . 
     The present invention provides a number of advantages. One such advantage is realized by monitoring the soft error rate and by taking action when appropriate in order to avoid hard errors that can result in the loss of user data. The action taken can be preventing a user from storing data in the system, permitting the user to store data only in a write-with-verify mode, or initiating recovery activity such as cleaning the read/write head. The soft error rate can be checked just after the head is loaded onto the disk, just before the head is unloaded from the disk, during a transfer of user data to or from the disk, or during an idle state of the system. 
     Yet another advantage is realized where the soft error rate is checked at a radius of the disk where data storage densities are highest and where error rate degradation is thus more likely to occur. Still another advantage is realized where the soft error rate is checked at two or more different radial locations on the disk. An advantageous alternative is to use a parameter other than the soft error rate, such as a channel quality parameter. 
     A different advantage is realized where, each time the head is moved to a position adjacent the disk, it is first moved to a position aligned with a predetermined portion of the disk where no user data is stored, and is maintained there for a period of time. It is also advantageous if the head is then subjected to a cleaning operation before it is moved to the portion of the disk where user data is stored. This head cleaning can be carried out unconditionally, or based on a condition such as whether there is an abnormality in an operational parameter derived from information read by the head from the disk. This operational parameter can be a position error signal derived from servo information read by the head from the disk. 
     A different advantage is realized where a procedure is provided to determine whether a degradation in system operation is due to a problem with the storage medium in the cartridge or a problem with the drive which includes the head. It is advantageous where the procedure involves evaluating a characteristic such as the operational spacing between the head and the disk at one or more locations, based on data and/or servo information read from the disk, and includes using that evaluation to determine what corrective action to take. The corrective action may involve rewriting data back to its current location on the disk, moving the data to a different location on the disk, replacing the cartridge containing the disk with a different cartridge and disk, and/or replacing the drive which includes the read/write head. 
     Although one embodiment has been illustrated and described in detail, it will be understood that various substitutions and alterations are possible without departing from the spirit and scope of the present invention, including aspects of the invention which are encompassed by the following claims.