Most Common Problems Incurred when using New Drives in machines with Older BIOS's

Most Common Problems Incurred
when using New Drives in machines with Older BIOS’s

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When you install a hard disk into a computer system that is larger than that which the BIOS is capable of handling, the system may react in a number of different ways, most of which are predictable. How a particular system responds though, will depend largely on the system, the BIOS date, and the overall quality of the BIOS routines. Most, if not all, issues noted below occur as the result of the hard disk having a total cylinder count larger than the maximum supported by the BIOS.

These are the four most common reactions you may expect with a machine having an older BIOS and a hard disk larger than it is capable of supporting, listed in the order of probability.

Truncation: A BIOS, when presented with a logical geometry containing more cylinders than it can handle, will simply truncate the total number of cylinders to the maximum it can support. You will usually experience this in an older BIOS that doesn’t support more than 1,024 cylinders, or in some cases in a BIOS with a set maximum of 4,096 cylinders. This is commonly found in systems that do not support Int13h extensions, as these systems typically see drives larger than 8.4 GB as being just 8.4 GB in size. Although truncation defeats the purpose behind adding a larger hard disk, the risk of losing data is minimal to nonexistent, and preferable to the other possibilities outlined below.


Many an old BIOS will presume that the number of cylinders in a drive will always be 1,024 or below, and therefore will only look to the bottom 10 bits of the cylinder number reported by the hard disk (2^10 = 1,024). As a result, when drives report cylinders over 1,023, the BIOS counts up to 1,024 and then wraps around to zero and starts over. As an example, let’s presume for the moment that you have purchased a brand new 60 GB hard drive for your computer. For the purposes of this example, we’ll presume this new drive was manufactured by Maxtor and it has 119,150 actual cylinders. Even in a recent manufactured computer you could only manually enter a maximum of 16,383 with current ATA specifications. In this scenario, the BIOS would only see 366 cylinders. This is because the BIOS would count up to 1,024 one hundred and sixteen (116) times to yield 118,784, or wrap around 116 times, ending with a net 366 cylinders (119,150 minus 118,784 = 366).

A nearly identical scenario can occur with a BIOS that supports only 4,096 cylinders, as it will only look at the bottom 12 bits. A troubling problem surfaced only a few years ago when computers began upgrading storage in older systems with small (by today’s standards) 2.5 GB hard disks only to learn that they had only about 400 MB of usable space showing up. Unfortunately, this was a common failure in systems that had a BIOS that didn’t support more than 4,096 cylinders.

Even some BIOS’s that support translation may wrap the cylinders if you disable translation. When you re-enable translation the problem may go away.

BIOS Ignorance: This is truly an ugly issue. Some systems with an older BIOS correctly reports the true number of logical cylinders of the drive, making you think the motherboard and BIOS (your system) supports the full size of the hard disk. In reality, the BIOS doesn’t have a clue as to the number of cylinders or what to do with them. It’s actually just reporting what the drive reports. When you attempt to partition and format the hard disk, you’re faced with the stark reality of a 1.024 cylinder limitation, but this is not readily evident. It can have you chasing your tail for hours trying to determine what is wrong. Fortunately though, this is only seen in systems with an older BIOS and the 1,024 cylinder limitation.

BIOS Failure: While you may not view it this way, a BIOS failure when attempting to install a large hard disk in a system with a BIOS that doesn’t support is probably a very fortunate situation. Obviously no one wants to deal with a barrier when upgrading their computer, but this particular failure might save you hours of searching for the reason for the failure. Many times and older BIOS will cause the system to completely lock up if you try installing a hard disk larger than can be supported. While fairly uncommon, it does occur with the more proprietary computer systems where the manufacturer withholds accurate BIOS and disk support information. Often you will experience this with some of the larger hard disk barriers and also with some of the more obscure ones.

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