ATAPI and the 137 GB limit

ATAPI isn't exactly common in the world of enterprise storage. It is an addition to the IDE standard (and part of EIDE) that allows devices such as tape drives to use ATA ports, so it's a backup enabler in certain situations. It is also still used on a lot of desktop machines as an inexpensive way of connecting tape drives and CD-ROMs.

Nearly all operating systems support ATAPI, but some versions of Windows, notably early versions of Windows 2000, Windows 98 and Windows Me, do not support large disk sizes under ATAPI. Specifically they don't support 48-bit logical block addressing called for in the latest versions of the ATAPI specification, which can cause data corruption and other problems with disks larger than 137 GB. As systems are upgraded piecemeal and disk sizes, and their related tape drives, increase, storage administrators may run into this combination.

Microsoft has provided a fix since Service Pack 3 on Windows 2000 and the equivalent service packs on the other operating systems. The upgrade requires that the system's BIOS be 48-bit LBA compatible and you must explicitly enable support by adding or changing the EnableBigLba value in the registry to 1.

Microsoft describes the situation in tech note 305098 titled "48-Bit LBA Support For ATAPI Disk Drives in Windows 2000" available at

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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.

This was first published in October 2003

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