Phantom drives and Data corruption under Windows 95 after installing >504MB harddisk

by Jan Steunebrink
Last updated on July 10, 2000

The problem

There is a Bug in Windows 95 that causes Data Corruption if All of the following conditions are true:
  1. The hard disk is a large (>504 MB) Enhanced IDE hard disk.
  2. The hard disk is accessed using Logical Block Addressing (LBA).
  3. The hard disk has been partitioned into multiple partitions with the Windows 95 version of FDISK, or another partitioning utility that creates the new partition types 0Eh and 0Fh.
  4. The system BIOS supports extended INT13h functions to access hard disk devices.
  5. You have exited to MS-DOS mode from Windows 95 without rebooting the computer.
Under these circumstances, certain data structures maintained by the operating system for accessing these logical drives become corrupted in the transition from protected mode to real mode.

This Bug is valid for All FAT-16 versions of Windows 95. (Win95 and Win95a)
Win95b (OSR2) should not be affected.
Windows 98 is free from this bug.

The solution

The Microsoft Knowledge Base article "Possible data Loss with LBA and INT 13 Extensions" (Q148821) addresses the problem. Here you will also read how to download and install the fix: The "Disk Type Specific Driver Update for Windows 95" (DSKTSUPD.EXE).

The key issue in this whole story is: Are INT 13h extensions present in my system BIOS?
I have written a utility that detects, and displays details of, the Int 13h extensions of your PC's BIOS.
If this utility can not find any extensions, then you're OK.
If it does, part of the display will look like this:

 Detection utility for Int 13h extensions  v1.3   (c) 1997-2000 Jan Steunebrink
 Harddisk 1: Int 13h extensions version 2.1 detected.
 Subset supported: Extended disk access support (for > 8GB) : Yes
                   Drive locking and ejecting support       : No
                   Enhanced Disk Drive (EDD) support        : Yes
 Device features : DMA boundary errors handled transparently: No
                   Device supports write with verify        : No
                   Media is removable                       : No
 Addressable geometry (CHS) is 524x255x63 Cylinders-Heads-Sectors/track
 A total of 8418060 Sectors of 512 Bytes (4110 MB) are addressable in LBA.

The BIOS may support one, two, or all three subsets of the Int 13h extensions. In case the 'Extended disk access' subset is supported, you may need the fix from Microsoft.

You can download the utility here; it is called EXTBIOS (6 KB ZIPfile).
The utility is now at version 1.3 (07/07/00) and comes with a text-file with more information.

Another useful program is Partition Information (127 KB) from PowerQuest.
It displays all data from the partition table(s) in readable form. Under 'FS' you can see the partition types, so you can check for the new partition types 0E and 0F.
If you see 0E or 0F instead of 06 or 05, you definitely need the fix from Microsoft.

Detailed information on the Int 13h extensions can be found at The Phoenix Technologies Technical Library. On this page you can download the specs-edd11.pdf file which contains the Enhanced Disk Drive specification v1.1 and the specs-edd30.pdf file that contains the EDD specification v3.0 update. The EXTBIOS utility is based on these documents.

The Int 13h extensions were designed to meet the ever increasing demand of harddisk space and performance. BIOSes with the extensions support:

The Int 13h extensions started to appear is 1995 and most BIOSes from 1998 or later will support the Int 13h extensions.
Windows 95 supports and uses the extensions if present.

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