With prudent preparation and upkeep, Emergency Repair Disks can fix damaged files that would otherwise kill your NT system.

Emergency Service

With prudent preparation and upkeep, Emergency Repair Disks can fix damaged files that would otherwise kill your NT system.

Anyone who’s installed Windows NT has seen the prompt, “Would you like to create an Emergency Repair Disk?” at the end of the installation process. Most people who are unsure of what this means dutifully place a disk in the drive and wait while the mysterious process completes. With the installation complete, the disk is usually put aside while the more pressing tasks of configuring the system are completed.

Exploring the ERD during an emergency is like buying automobile insurance while viewing the remains of your car. Even for those of you who know what the ERD is, how many of you have actually taken the time to see if it works? Do you know the specific steps necessary to use it? If not, you’re not alone—so read on and sharpen another arrow for your quiver.

ERD Basics

The general rule of thumb is that if there’s a configuration problem and you haven’t logged onto the machine, try the LastKnownGood option during boot time. If this doesn’t work or if you have a corrupted system file on the machine, you should use the ERD process. If this last-ditch effort fails, it’s likely that you’ll have to reinstall a new system and restore the original system from backup.

The ERD shouldn’t be considered a replacement for backup—it’s a repair utility. The ERD is a machine-specific disk that contains copies of certain Registry hives, system files, and a log of the installation process created during the setup process. The files on the ERD are copied from the ones that are located in the \%Systemroot%\Repair directory. On a Windows NT 4.x machine these files are:

  • AUTOEXEC.NT initializes the MS-DOS environment and is located in \%Systemroot%\System32\.
  • CONFIG.NT also initializes the MS-DOS environment and is located in \%Systemroot\System32\.
  • DEFAULT._ is the compressed Registry hive HKEY_USERS\DEFAULT.
  • NTUSER.DA_ is the compressed Registry hive located in %Systemroot\Profiles\DefaultUser\Ntuser.dat.
  • SAM._ is the compressed Registry hive HKEY_LOCAL_MACHINE\SAM, which contains the user account information for this machine.
  • SECURITY._ is the compressed Registry hive HKEY_LOCAL_MACHINE\SECURITY, which contains the security context for the users on the machine.
  • SETUP.LOG lists the files that were installed and the CRC information that’s used during the repair process. This file is updated as configuration changes are made to the machine.
  • SOFTWARE._ is the compressed Registry hive HKEY_LOCAL_MACHINE\SOFTWARE, which contains configuration information for the applications installed on the machine.
  • SYSTEM._ is the compressed Registry hive HKEY_LOCAL_MACHINE\SYSTEM, which contains configuration information for the services installed on the machine.

This brings up an important point: When you create an ERD, you’re actually copying these files from the \%Systemroot%\Repair directory to a 3.5-inch disk. It’s possible that this directory contains out-of-date information or that the ERD contains different information than the %Systemroot\Repair directory, as shown in Figure 1 by the different dates for the files.

Figure 1. When you create an ERD, you’re actually copying these files from the \%Systemroot%\Repair directory to a floppy disk. It’s possible that this directory contains out-of-date information or that the ERD contains different information than the %Systemroot\Repair directory, as shown here by the different dates for the files.

The reason many ERDs haven’t been updated is because there’s no icon to the utility that manages the ERD built during installation. This utility is called RDISK.EXE and is located in \%Systemroot\System32. As with any program, you can create an icon for the program or you can simply enter RDISK.EXE at the command prompt, which will bring up the screen in Figure 2.

Figure 2. The utility that manages the ERD built during installation is called RDISK.EXE, and it’s located in \%Systemroot\System32. Entering RDISK.EXE at the command prompt will bring up this screen.

Exit and Help are self-explanatory, but let’s look at the other two options. Create Repair Disk will copy the files that are currently in the \%Systemroot\Repair to the ERD. Update Repair Info will compile the current system configuration information from the Registry and create new files that reflect this information in the \%Systemroot\Repair directory. This option will also ask if you want to update the ERD as well. If you follow these prompts, you’ll notice that the dates on the ERD will match the dates of the files in the \%Systemroot\Repair directory (see Figure 3).

Figure 3. RDISK.EXE will update the ERD so that the dates on the ERD match the dates of the files in the \%Systemroot\Repair directory.

It’s important to update the ERD every time you make significant changes to a machine’s configuration. This brings up another issue: you’ll notice that only some of the Registry hive files have been updated. This is because on some machines, notably Domain Controllers, this information can consume many megabytes of space, well beyond the space limitations of a 3.5-inch disk—and an ERD can’t span disks. For this reason you have to take an extra step to update the user and security information in the %Systemroot\Repair directory. That extra step is to use an /S switch when you run RDISK.EXE. This will update the other Registry hives as shown in Figure 4. Also note that when RDISK /S is run at the command prompt, the Repair Disk Utility screen won’t be displayed. And finally, you should document what administrator password(s) were at the time that the recovery disk were made, in case you need to repair the SAM. Otherwise, life can get very interesting trying to remember old passwords.

Figure 4. Adding an /S switch when you run RDISK.EXE will update the other Registry hives.

Creating the ERD

Now that we’ve discussed the various alternatives for creating or updating a new ERD, let’s step through the emergency repair process. The ERD isn’t a bootable disk. Rather, it’s used with the NT setup program. The setup program is run by using the three setup disks, which you can re-create using the winnt32 /ox command from the NT CD. Place disk 1 in the machine, or boot from the NT CD if possible, and reboot. This will load some system files and then request disk 2. You’ll reach a screen that asks if you want to update a current installation or create a clean install. At the bottom of this screen, there’s also an option to press R—Repair. This will bring up the following text:

As part of the repair Process, Setup will perform each of the optional tasks shown below with an "X" in its check box.

To perform the selected tasks, press ENTER to indicate "Continue." If you want to select or deselect any item in the list, press the UP or DOWN arrow key to move the highlight to the item you want to change.

Then press ENTER.

[X] Inspect Registry files.
[X] Inspect startup environment.
[X] Verify Windows NT system files.
[X] Inspect Boot Sector.
Continue

F1=Help F3=Exit ESC=Cancel
ENTER=Select/Deselect

  • Inspect Registry files attempts to load each selected Registry hive from a second screen and determines if it’s corrupted. If so, it gives the administrator the opportunity to replace it with the copy on the ERD or leave it alone.
  • Inspect startup environment verifies that the system partition files such as NTLDR are present. If they’re missing or corrupt, they’re replaced from the Windows NT installation CD.
  • Verify Windows NT system files uses a checksum to verify that all of the files are good by matching them with the files on the installation CD. If any of the files don’t match, then you’re given an opportunity to replace them from the CD.
  • Inspect Boot Sector checks the boot loader functionality, which calls NTLDR and replaces it if necessary from the ERD.

You can select one or all of these functions by using the up and down arrow keys and pressing enter to select and de-select the options. After this screen, the normal setup screens follow and display information regarding the mass storage devices and SCSI adapters, and it gives you the opportunity to specify additional adapters. Windows NT 4.x requires the installation CD to continue and will request it if you don’t have it in the drive. (Refer to KnowledgeBase article Q150497 on How to Repair Windows NT System Files Without a CD-ROM Attached if you don’t have a CD-ROM available on the server.) You’ll then be prompted for an ERD or be asked if you want setup to locate the files on the local hard drive, which will be the \%Systemroot\repair directory. Regardless, if you’ve chosen the Inspect Registry files option, the next screen will present some more detailed options for the recovery process:

Setup will restore each registry file shown below with an "X" in its check box.

To restore the selected files, press ENTER to indicate "Continue." If you want to select or deselect any item in the list, press the UP or DOWN ARROW key to move the highlight to the item you want to change. Then press ENTER.

WARNING: Restore a registry file only as a last resort. Existing configuration may be lost. Press F1 for more information.

[ ] SYSTEM (System configuration)
[ ] SOFTWARE (Software Information)
[ ] DEFAULT (Default User Profile)
[ ] NTUSER.DAT (New User profile) (NT V4.0 only)
[ ] SECURITY ( Security Policy) and SAM (User Accounts Database)
Continue (perform selected tasks)

F1=Help ENTER=Select/Deselect F3=Exit

If you select any of these options, the Registry hives will be checked for corruption. You’ll be given the option to replace any corrupt files with the copy on the ERD or in the \%Systemroot\Repair directory.

If you select the Verify Windows NT system files option, the files on the hard drive are compared with the files listed in the SETUP.LOG file on the ERD (see Figure 5).

Figure 5. If you select the Verify Windows NT system files option, RDISK.EXE will compare the files on the hard drive with the files listed in the SETUP.LOG file on the ERD..

As you can see, the location path is listed along with the checksum values that are used for verification. If these differ from the files on the CD, you’ll have the option of individually replacing the file. You can also select an option to have any file with a checksum mismatch be automatically replaced. After the ERD process is complete, you’ll be prompted to remove any disks and CDs from the drives and reboot to bring the system up... hopefully.

Coming Next Month...

The ERD process provides a reliable and straightforward method for repairing files that would otherwise prevent your NT system from booting. There are still several issues that prevent the widespread use of the ERD. For one, in an installation of thousands of NT workstations it can be very time-consuming to keep all the ERDs up to date. For this reason, many sites just use the ERD process for Domain Controllers. Next month, I’ll explore some of the deeper details surrounding the ERD and some methods for solving or working around those issues.

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