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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk BIOS and Capacity Factors | Overcoming BIOS Disk Size Barriers ]

Disk Size Reduction Jumpers

A rather sub-optimal solution that is offered as an alternative by some hard disk manufacturers is the disk size reduction jumper. Certain hard disk size barriers can cause a hard drive to not be seen by a system at all. If you set one of these jumpers on the hard disk, this tells the disk to change the drive parameters it presents to the system, reducing the size of the drive. Since the system sees the drive as being small enough to avoid the size barrier, the barrier is avoided. Of course, this costs you capacity: you lose the storage of the drive above the size barrier, unless you supplement the jumper with a software overlay.

The first time size reduction jumpers were commonly used was to get around the 4,096 cylinder barrier. One of the common ways that a BIOS with this barrier may treat a disk that is over about 2.1 GB is wrapping around. So a 2.5 GB disk would be seen as only around 400 MB. The size reduction jumpers would cause the hard disk to "pretend" that it had less than 4,096 cylinders, so it was only 2.1 GB in size. This is of course a waste of however much space the disk really has over 2.1 GB, but it is better than only using 400 MB of the disk.

Similarly, some newer drives may come with size reduction or capacity limiting jumpers to get around the 32 GB size barrier. These force the drive to present a size small enough to avoid triggering that capacity barrier.

This "solution" is in some ways the hard disk equivalent of that old Marx Brothers routine: "Doctor, it hurts when I do this"... "So don't do that!" ;^) It's not really a solution, but rather a way to avoid the consequences of the problem. It's better than having your PC hang when you try to boot the system, but still, this solution is a very poor one. The only time these reduction jumpers really make sense is when you use them in conjunction with drive overlay software--in some cases, without the jumpers you may not even be able to get the system booteed so you can install the overlay! Proper hardware support is still a better solution of course.

Next: Manually Entering Drive Geometry Parameters


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