Upgrading Pre-4.0 Systems with Logical Drive(s) > 32 MB

Some original equipment manufacturers (OEMs) modified their adaptations of MS-DOS to provide logical drives larger than 32 megabytes in versions of MS- DOS earlier than version 4.0. This was usually implemented by using a sector size larger than the standard value of 512 bytes. This is called "logical sectoring" because the sector size of the drive is a logical value; that is, it is dependent on the value in the BIOS parameter block of the drive for bytes_per_sector. Microsoft MS-DOS versions 4.0 and later do NOT support this logical sectoring on hard disk drives. A sector size of 512 bytes per sector is assumed on all hard disk drives.

If you have used a version of MS-DOS that uses logical sectoring to create a logical drive larger than 32 MB and want to upgrade to MS-DOS 4.0 or later, you MUST either repartition the disk or use the Microsoft MS-DOS Upgrade SETUP.EXE to convert the hard disk drive.

WARNING: After you upgrade the hard disk drive, booting the previous version of MS-DOS and writing information to the hard disk drive may result in data corruption. Returning to the previous version of MS-DOS requires either repartitioning or using the MS-DOS Upgrade Uninstall feature.

NEC, Toshiba, Wyse, and Zenith are known to use logical sectoring, and other OEMs may have done this as well. Compaq MS-DOS 3.31 is known to have implemented large logical drives in the same way as Microsoft MS-DOS 4.0 and later (that is, without logical sectoring) and therefore is version 4.0 compatible.

To test whether the new version of MS-DOS correctly accesses the disk, boot the new version and run CHKDSK (not CHKDSK /F). You should get the same output as the current MS-DOS.

Repartitioning deletes all information stored on the disk. It may be necessary to delete the current partitions with the current version of MS- DOS Fdisk. The new partitions MUST be created with the new version of MS- DOS Fdisk.

The Microsoft MS-DOS Upgrade SETUP.EXE converts all MS-DOS compatible partitions with logical sectors into standard 512-byte sectors. After you upgrade the hard disk drive, booting the previous version of MS-DOS and writing information to the hard disk drive may result in data corruption.

The SETUP.EXE file that ships in Microsoft MS-DOS versions 5.0 and later (generic OEM or packaged product) does not convert partitions with logical sectoring into standard 512-byte sectors. Repartitioning can only be avoided by using the Microsoft MS-DOS Upgrade SETUP.EXE.

To return to the previous version of MS-DOS after upgrading using Microsoft MS-DOS Upgrade SETUP.EXE, the best option is to use the Uninstall program. UNINSTAL.EXE replaces the previous MS-DOS version and converts the large partitions to their previous states. To use UNINSTAL.EXE to return to the previous MS-DOS version, boot from the Uninstall #1 disk created while upgrading to MS-DOS 5.0 or later.

NOTE: If DELOLDOS has been run, Uninstall cannot return the drive to its pre-upgrade state. Returning to the previous version of MS-DOS requires repartitioning, which destroys all information stored on the drive.

For more information on Uninstall, query on the following words here in the Microsoft Knowledge Base: