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Solving disk size limits on X86 systems

By Rick Cook

If an X86 system hangs on boot up, one of the most common causes is a partition that is too large, especially the boot partition. Often the culprit is a boot partition larger than 1024 cylinders on a Windows X86 system using a SCSI controller. In addition, various other Microsoft and non-Microsoft operating systems have built-in limits on the size of partitions they can handle.

According to Microsoft (Q127134 in the Microsoft support knowledge base, the boot partition problem in Windows NT operating systems has to do with the limits of the X86 BIOS. On an X86, the BIOS INT 13 calls will not recognize a disk with more than 1024 cylinders. Since this is a hardware or firmware limit, NT can't fix it, Microsoft says. The company says 1024 cylinders usually translate into a boot partition of between one and two G Bytes.

The Microsoft-preferred workaround is to use a SCSI host adapter that translates disk size information before the BIOS ever sees it, and to repartition the disk. Many newer SCSI controllers can do this automatically if the controller translation feature is enabled.

A simple fix on an older controller is to create a small boot partition for the OS.

This is only one of a number of problems created by interactions between hardware/firmware, multi-gigabyte drives and the limits and various operating systems. According to disk-drive manufacturer Maxtor, (http://www.Maxtor.com), there are capacity limits at 2.048 G Bytes, 4.2 G Bytes, 8.4 G Bytes and 32 G Bytes which can cause trouble with various hardware/BIOS/OS combinations. Besides DOS and Windows, these problems can affect Novell NetWare, OS/2 and Linux. In most cases, patches and workarounds are available from the OS manufacturers.

Maxtor has a question-and-answer document titled, "Breaking the Barriers" on its Web site, which explains these limits and the fixes in some detail, and is aimed at EIDE interfaces, rather that SCSI.

Rick Cook has been writing about mass storage since the days when the term meant an 80K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.

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