News & Views

It has happened to many of us, and is not a pleasant experience: the computer starts to behave strangely and some files seem to have mysteriously disappeared.
If your computer is showing these symptoms, it might be suffering from a damaged file system on its hard disk.
There can be many causes of this, including faulty hardware in your computer or a power cut at a critical point while the disk is being updated, but the most common cause is software bugs, either in the operating system itself or in an application or other piece of software on the computer.
If you think the file system on your hard disk might be damaged, you should stop using the computer as soon as possible, as any data written to your hard disk might cause further damage to the file system, possibly resulting in loss of files. Use disk repair software to check for and repair any problems before you resume normal operation. Mac OS X has improved the reliability of the file system considerably over Mac OS 9, simply by preventing applications from being able to write data to arbitrary locations on the hard disk, and from overwriting critical system files. In addition, Mac OS X 10.3 (and later) supports a scheme known as journalling, which allows it to recover from the computer crashing, restarting or losing power, restoring the file system to a consistent state. Unfortunately this system isnt perfect, and it is still possible to get minor problems occurring on rare occasions. Sometimes these minor problems can grow into major ones, and once the file system has major damage, you are at risk of losing a lot of files on your hard disk. So, how does one deal with these problems, and hopefully catch them before they get too serious?

Disk Utility: First Aid

The primary tool on Mac OS X is the Disk Utility application. It includes a First Aid component, which can check your hard disk for any file system damage, and repair simple problems. It is a good idea to regularly run Disk Utility and do a Verify on your computers hard disk to check for any problems. Hopefully this will catch any minor issues before anything serious happens. If you are running Mac OS X 10.3 or later, Id recommend doing this every few months. You should do it more often if you are running an earlier version of Mac OS X.
Unfortunately, you cant use the Verify (or Repair) function of Disk Utility unless you start up your computer from another disk. Apple have provided a method of doing this, by including a copy of Disk Utility on the Mac OS X installation CD/DVD which was supplied with your computer (or a retail copy purchased separately). They hid it a little, so you need to know where to look in order to find it. Insert your Mac OS X installation CD/DVD into the drive, and run the Mac OS X Installer. If you have more than one such CD, make sure you use the latest one, e.g. a 10.4 (Tiger) install CD should be used in preference to a 10.3 (Panther) one.
This will involve restarting the computer from the CD (which is done automatically when you run the installer by double-clicking its icon). The Mac OS X installer starts by asking some introductory questions such as which language you want to use. After the first question or two you should see a standard Mac OS X menu bar at the top of the screen, with a blue apple menu, and an Installer menu. If you look in the Installer menu, you will find a Disk Utility item. Select this to run Disk Utility.
In Disk Utility, you need to select your hard disk from the list on the left side, then make sure you have the First Aid tab selected, and click on the Verify button.
This will check your disk for any problems. It displays progress information and a summary of what it found. If there are any problems, they will be shown in red, and it will say that you need to repair the disk. You can do this by clicking on the Repair button. Assuming the problem was minor, Disk First Aid should be able to fix it. When finished, you can quit from Disk Utility, and quit from the Installer (you dont need to proceed with reinstalling Mac OS X), then restart your computer to return to normal operation.

Third-party Disk Repair Tools

If there is a serious problem, Disk First Aid may not be able to repair it. This is when you need to call in the heavy guns, in the form of specialist third-party disk repair programs. These are all relatively expensive, and it is important to ensure you are running the latest version. There have been cases where an old version of a disk repair utility has done serious damage to a file system which has been modified by newer system software.
In the past, the main disk repair was Symantecs Norton Utilities. This program is no longer being developed, and more importantly the latest version is not compatible with Tiger (10.4), so I cannot recommend its use, and I would discourage anyone from buying it now. In my experience, the best program for dealing with a corrupted file system is Alsofts DiskWarrior. It takes a while to rebuild the directory on your hard disk, then gives you the option of previewing its results (including being able to copy files elsewhere, such as an external hard disk) before it actually modifies your hard disk. If it has not succeeded in recovering files you know were lost, you can cancel DiskWarriors repair attempt, and try other repair tools first. DiskWarrior is certainly a lot easier to use than Norton Utilities, as it doesnt ask mysterious technical questions whenever it encounters a problem.
Another major disk repair program is TechTool Pro, which also provides a wide range of other tests for your computer hardware. I have this, but Ive never needed to use its disk repair functions because DiskWarrior has always done a good enough job, so I cant comment on its reliability or usability.
Another useful program is Data Rescue. This concentrates on recovering files from a damaged hard disk, rather than trying to repair it. If you have files on your hard disk which have been lost, are absolutely critical and you must recover them at all costs, Id recommend trying this one before anything else, though I havent had need for it myself. All of these programs cost around US$80 to US$90.

The Aftermath

If you were suffering from major damage to your file system, then you may find that the computer is still misbehaving after the damage has been repaired.
The most likely explanation is that some critical system files were partially damaged, and they need to be reinstalled. You should try doing a fresh installation of your operating system, and preferably all your applications as well, or restore everything from a known good backup.

Optimization?

Some disk tools offer a method of optimizing your hard disk in order to improve performance.
If you are running Mac OS X, Id recommend avoiding this. The speed gains are temporary, and the optimization process is risky: if the file system is already damaged, optimizing it is likely to cause serious damage.

David Empson is currently the President of the Wellington Macintosh Society in New Zealand and works as a full time computer programmer.