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Fix Hard Disks Running
in MS-DOS Compatibility Mode

A HelpWithWindows TechFile

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If you view the information on the Performance tab (Control Panel > System), you may see a message:

Compatibility Mode Paging reduces overall system performance

Note: the message:

Some drives are using MS-DOS compatibility

...means that your floppy disk drive or CD-ROM drive are operating in MS-DOS compatibility mode. We discuss only hard disks operating in MS-DOS compatibility mode here.


To force Windows to attempt to reinitialize the protected-mode IDE driver you'll have to remove the noide entry from the registry:

  1. Start the Registry Editor
  2. Go to HKEY_LOCAL_MACHINE \ System \ CurrentControlSet \ Services \ VxD \ IOS
  3. Delete the NoIDE entry
  4. Search your registry for BAD_IDE (You can Search the registry by pressing F3 while working in the registry editor.
  5. Delete any BAD_IDE values you find

After you update the registry, restart Windows. Windows will then attempt to initialize the protected-mode driver for the controller. If no problems are encountered, the file system and virtual memory will operate in 32-bit mode, and Device Manager will not display an exclamation point in a yellow circle for the IDE channels.

If the protected-mode driver is not initialized properly, an error message will be displayed and the NoIDE registry entry will be re-created.


There are several reasons why your Hard Disks might be running in MS-DOS compatibility mode:

  • An "unsafe" device driver, memory-resident program, or virus hooked the INT21h or INT13h chain before Windows loaded
  • The hard disk controller in your computer was not detected by Windows
  • The hard disk controller was removed from the current configuration in Device Manager
  • There is a resource conflict between the hard disk controller and another hardware device
  • The Windows protected-mode driver is missing or damaged
  • The Windows 95 32-bit protected-mode disk drivers detected an unsupportable configuration or incompatible hardware

Microsoft released an update for Windows 95 OSR 2 & OSR 2.1, to fix a problem that would stop the computer from responding (hang) while the hard disk was being accessed, when using an IDE (ATA) hard disk and controller that support Ultra DMA mode.

To install this update, follow these steps:

  1. Download the Remideup.exe [143KB] file to an empty folder
  2. In Windows Explorer, double-click the Remideup.exe file you downloaded in step 1
  3. Follow the instructions on the screen

Microsoft lists a number of Knowledge Base Articles on this subject:


Last Updated: February 15, 2005