One of the most reassuring features in the Windows XP operating system is System Restore. To do its job, System Restore monitors the status of your computer and looks for any operations—such as the installation of an application, an operating system update, or a device driver—that changes your system configuration. If System Restore detects such an operation, it will create a restore point, which is a backup of your current system configuration, and save it to the hard disk. This means that if an installation operation causes a system problem, you can use the System Restore utility to return your computer to the way it was just before the problem occurred—all without losing any of your crucial data files and settings.

However, it’s possible to encounter the following error message if you try to restore your system with the System Restore utility after experiencing such a disaster:
System Restore is suspended because there is not enough disk space available on the system drive (drive letter). To restart System Restore, ensure at least 200 megabytes (MB) of free disk space are available on this drive.
Do you want to start Disk Cleanup to free more disk space now?
Yes No

As you can see, this message indicates that the System Restore utility has been disabled due to the lack of available space on the hard disk. Unfortunately, this means that System Restore was unable to create a restore point before the problem occurred. While this message offers to run Disk Cleanup to rectify the situation, it’s too late to really help you. Wouldn’t it be nice to have known about this problem beforehand?

Although the operating system will warn you about low disk space, it won’t tell you that System Restore has been suspended. In this Daily Drill Down, I’ll take a look at how the System Restore utility uses hard disk space. As I do, I’ll point out some problem areas and explain how you can avoid them.

System Restore uses a hidden system folder
As I mentioned, the System Restore utility creates its restore points on the hard disk. To do so, it uses a special hidden system folder, called System Volume Information, on the root of the disk. This system folder is completely inaccessible using Windows Explorer, since tampering with its contents could cause all sorts of problems. In fact, if you double-click the folder, you’ll see the error message shown in Figure A. Furthermore, if you right-click the System Volume Information folder and select Properties, you’ll discover that the operating system reports that this folder is basically empty.

Figure A
If you try to open the System Volume Information folder from within Windows Explorer, you’ll receive an error message.

Gaining access to the System Volume Information folder

If you really want to take a look the contents of the System Volume Information folder, there are several methods for doing so. Each of these methods is explained in the Microsoft Knowledge Base article Q309531.

As you can imagine, each one of these restore points can be pretty substantial in size. By default, System Restore is configured to allocate 12 percent of the available space on the hard disk for the creation of restore points. However, the actual amount of hard disk space that this 12-percent figure represents will vary depending on the total size of the hard disk. For example, 12 percent of the available hard disk space on my laptop system’s 6-GB partition is 718 MB. On the other hand, 12 percent of the available hard disk space on my desktop system’s 80-GB hard disk is 9 GB.

The indirect warning signs
Now that you understand how dependent the System Restore utility is on hard disk space, you can probably see that a lack of disk space is the most common problem that can prevent the System Restore utility from functioning correctly. Let’s take a closer look.

When the amount of available hard disk space falls below 200 MB on the main hard disk, a Low On Disk Space message will pop up from the notification area of the taskbar, like the one shown here:
Your Windows drive has only (amount MB) of free disk space available.
It is recommended to have at least a minimum of 200 MB free on your Windows drive to ensure system reliability. You can free even more disk space by removing programs that you do not use.
Would you like to do that now?

As soon as this message appears, the operating system automatically suspends the System Restore utility. In other words, it will no longer create any restore points. However, if you look closely at this warning message, you’ll see that there’s no mention that System Restore has been suspended.

If you click on the Low On Disk Space message, the Disk Cleanup utility will automatically launch and prompt you to free up disk space. If you use the Disk Cleanup utility and free up more than 200 MB of space on the hard disk, the System Restore utility will automatically resume operation.

The problem escalates
Now, if you’re not really worried about hard disk space and choose to ignore this warning message, you’d never know that System Restore is no longer functioning. Nor would you be notified that System Restore is suspended if you run the Disk Cleanup utility but don’t free up more than 200 MB of space on the hard disk.

If you don’t remedy the available disk space problem and the amount of available hard disk space falls below 80 MB, System Restore will automatically begin purging the oldest restore points. Once the amount of available hard disk space falls below 50 MB, System Restore will automatically purge all of the restore points.

The solution? Free up more disk space
Obviously, the best way to avoid this problem is to make sure that you have more than 200 MB of available space on your main hard disk. Furthermore, any time you receive a Low On Disk Space warning message and then run the Disk Cleanup utility, you should check the System Restore settings and make sure that the System Restore utility is again able to monitor your hard disk.

By default, System Restore will wait for 15 minutes after you’ve freed up at least 200 MB of space on the hard disk before it automatically resumes monitoring. However, you can resume monitoring by immediately by running the System Restore utility to create a new restore point.

The secondary hard disk problem
You can also have problems with System Restore even if you’re diligent about keeping adequate space available on your main hard drive. If you have more than one hard disk in your system, System Restore will suspend operations if available space on the secondary hard disk falls below the 50-MB minimum. Again, you won’t receive any specific error message indicating that System Restore is no longer working.

The reason a secondary hard disk can cause System Restore to suspend operation is that, by default, System Restore is configured to monitor all the hard disks installed in your computer. This means that if you have two hard disks in your computer—e.g., a main hard disk that has Windows XP installed on it and another hard disk that is used exclusively for data storage or backups—System Restore monitors both hard disks.

When the amount of available space on a secondary hard disk that System Restore is monitoring drops below 50 MB, System Restore will automatically suspend operations on all hard disks—even though there may be plenty of available space on the main hard disk. The reason for this is that the System Restore utility takes into account the possibility that the secondary hard disk could contain applications whose settings are stored in the registry on the main hard disk. If this were the case and System Restore only suspended operations on the secondary hard disk, running System Restore to restore your PC would throw the whole system out of whack.

The solution? Disable System Restore on additional hard drives
If you have a secondary hard disk in your system that you use exclusively for data storage or backup, you should disable System Restore on that drive. To do so, access the System Properties dialog box and select the System Restore tab. Select the secondary hard disk in the Available Drives list, as shown in Figure B. Next, click the Settings button.

Figure B
If you have more than one hard disk in your system, System Restore will be configured to monitor all the hard drives in your computer.

When you see the Settings dialog box for the secondary hard disk, select the Turn Off System Restore On This Drive check box, as shown in Figure C. When you click OK, you’ll see a confirmation dialog box. Click Yes to complete the operation. When you return to the System Restore tab, you’ll see that the utility is no longer monitoring your secondary hard disk.

Figure C
You can disable System Restore for a secondary hard disk.

More information
If you want to learn more about how to use System Restore, you should investigate the article “Recover from a crash with XP’s System Restore” by Steven Warren. For more information on Disk Cleanup, check out the article “Teach users to keep their hard drives clean with Disk Cleanup” by Brien Posey.