Fix Hard Disks Running
in MS-DOS Compatibility Mode
A HelpWithWindows TechFile
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:
- Start the Registry Editor
- Go to HKEY_LOCAL_MACHINE \ System \ CurrentControlSet \ Services \ VxD \ IOS
- Delete the NoIDE entry
- Search your registry for BAD_IDE (You can Search the registry by pressing F3 while working in the registry editor.
- 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:
- Download the Remideup.exe [143KB] file to an empty folder
- In Windows Explorer, double-click the Remideup.exe file you downloaded in step 1
- Follow the instructions on the screen
Microsoft lists a number of Knowledge Base Articles on this subject:
- 82923 - Methods to Detect a Boot-Sector Virus
- 126855 - Windows Support for Large IDE Hard Disks
- 130179 - Troubleshooting MS-DOS Compatibility Mode on Hard Disks
- 136742 - Drives Use MS-DOS Compatibility Mode with Iomega Zip Drive
- 137405 - Mh32bit.386 Device Driver Causes MS-DOS Compatibility Mode
- 151911 - MS-DOS Compatibility Mode Problems with PCI-IDE Controllers
- 152701 - FAT32 Using Compatibility Mode with OnTrack Disk Manager
- 153164 - MS-DOS Compatibility Mode on National Semiconductor Controller
- 163317 - MS-DOS Compatibility Mode When Booting from Removable Disk
- 175629 - Large IDE Hard Disk May Run In Compatibility Mode
- 242607 - Performance and MS-DOS Compatibility Mode Issues with VIA Busmaster Controller