DISK 63 SECTOR LIMIT

5/15/2004

Older IDE drives were often configured using the actual number of cylinders, heads, sectors; or with a manufacturer suggested configuration; or with the nearest smaller preset "type". If they didn't happen to be configured with 63 sectors per track, they may not be readable in many modern PCs whose disk address translation algorithms assume 63 sectors per track.

When IDE disks first appeared, they were configured using the actual disk format recommended by the manufacturer. Unlike the MFM drives they replaced, it wasn't really necessary to do that as long as the configuration used to set up the disk had no more sectors than the disk would hold. In the 1990s, disks appeared that didn't actually have the same number of sectors on all tracks. As disks pushed beyond their initial addressing limits, it became common practice to set sectors per track to the maximum value allowed by Interrupt 15 and the IDE addressing used to actually access the disk -- 63. In the late 1990s, drives appeared that required translations in the BIOS to convert between INT15 and IDE addressing. Some of these algorithms -- especially an algorithm called "Assisted LBA" assume 63 sectors per track.

While it doesn't matter what configuration is used to configure a disk, it is necessary to use the same configuration to access the disk that was used to do high level formatting. If the configuration does not use 63 sectors per track, then the disk may not be readable in a PC that assumes 63 sectors per track until the disk is reformatted. In many cases, this can be overcome by manually specifying the proper configuration. Some BIOSes will not allow manual configuration. In those cases, the drive will not be usable on that PCs until it is reformatted specifying 63 sectors per track. Reformatting will effectively erase any data on the drive.

If the reason for transplanting the drive is to move data, some other approach will have to be found. Perhaps the data can be transferred in a different computer. Or networking can be used. Or CDROM or DVD.

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