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After five retries, the DOS FORMAT program gives up on this track or sector and moves to the next. Areas that remain unreadable after the initial read and the five retries are noted in the FAT as being bad clusters. DOS 3.3 and earlier versions can mark only entire tracks bad in the FAT, even if only one sector was marked in the LLF. DOS 4 and later versions individually check each cluster on the track and recover clusters that do not involve the LLF marked-bad sectors. Because most LLF programs mark all the sectors on a track as bad, rather than the individual sector that contains the defect, the result of using DOS 3.3 or 4 is the same: all clusters involving sectors on that track are marked in the FAT as bad.

Some LLF programs mark only the individual sector that is bad on a track, rather than the entire track. This is true of the IBM PS/2 low-level formatters on the IBM PS/2 Advanced Diagnostics or Reference disk. In this case, high-level formatting with DOS 4 or later versions results in fewer lost bytes in bad sectors because only the clusters that contain the marked bad sectors are marked bad in the FAT. DOS 4 and later versions display the Attempting to recover allocation unit x message (in which x is the number of the cluster) in an attempt to determine whether a single cluster or all the clusters on the track should be marked bad in the FAT.

If the controller and LLF program together support sector and track sparing, the high-level format finds the entire disk defect-free because all the defective sectors have been exchanged for spare good ones.

If a disk has been low-level formatted correctly, the number of bytes in bad sectors is the same before and after the high-level format. If the number does change after you repeat a high-level format (reporting fewer bytes or none), the LLF was not done correctly. The manufacturer's defects were not marked correctly, or Norton, Mace, PC Tools, or a similar utility was used to mark defective clusters on the disk. The utilities cannot mark the sectors or tracks at the LLF level. The bad-cluster marks that they make are stored only in the FAT and erased during the next high-level format operation. Defect marks made in the LLF consistently show up as bad bytes in the high-level format, no matter how many times you run the format.

Only an LLF or a surface-analysis tool can correctly mark defects on a disk; anything else makes only temporary bad-cluster marks in the FAT. This kind of marking may be acceptable temporarily, but when additional bad areas are found on a disk, you should run a new LLF of the disk and either mark the area manually or run a surface analysis to place a more permanent mark on the disk.

Hard Disk Drive Troubleshooting and Repair

If a hard disk drive has a problem inside its sealed HDA, repairing the drive usually is not feasible. If the failure is in the logic board, you can replace that assembly with a new or rebuilt assembly easily. However, unless you have data you need to recover, it makes more sense to just buy a new drive, considering today's cost.

Most hard disk problems really are not hardware problems; instead, they are soft problems that can be solved by a new LLF and defect-mapping session. Soft problems are characterized by a drive that sounds normal but produces various read and write errors.

Hard problems are mechanical, such as when the drive sounds as though it contains loose marbles. Constant scraping and grinding noises from the drive, with no reading or writing capability, also qualify as hard errors. In these cases, it is unlikely that an LLF will put the drive back into service. If a hardware problem is indicated, first replace the logic-board assembly. You can make this repair yourself and, if successful, you can recover the data from the drive. If replacing the logic assembly does not solve the problem, contact the manufacturer or a specialized repair shop that has clean-room facilities for hard disk repair.

Most of the time, a seek failure indicates that the drive is not responding to the controller. This failure usually is caused by one of the following problems:

  Incorrect drive-select jumper setting
  Loose, damaged, or backward control cable
  Loose or bad power cable
  Friction between drive heads and platters
  Bad power supply

If a diagnostics cylinder read error occurs, the most likely problems are these:

  Incorrect drive-type setting
  Loose, damaged, or backward data cable

The methods for correcting most of these problems are obvious. If the drive-select jumper setting is incorrect, for example, correct it. If a cable is loose, tighten it. If the power supply is bad, replace it. You get the idea.

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