If there is a problem with a disk's directory structure, the disk may appear intermittently or not at all in Mac OS 9 or Mac OS X, a message may appear indicating that the disk is not readable, or other issues may appear when an attempt is made to access parts of the disk. There are a few applications and other methods of dealing with this issue:

1. Mac OS 9 and earlier includes the Disk First Aid application, and Mac OS X provides the Disk Utility. Both of these applications have some testing and repair capabilities. To use these applications, open the Applications or Applications (Mac OS 9) folder and then the Utilities folder inside. One of these applications should be present on any configuration, and should be opened once it is found. If the Disk First Aid application is present, select the disk in question from the list at the top and then click on the Repair button. For the Disk Utility, click on the disk in the sidebar to the left, and then on the Repair Disk button. These utilities cannot fix every issue that may be experienced with a disk.

2. Micromat sells an application called TechTool Pro, which has more capabilities and can also check other parts of a computer. The purchase of AppleCare for a Mac includes a CD or a downloadable image of a CD with TechTool Deluxe installed, which shares many of the same capabilities. In either case, look at the Volume Structures section of TechTool for the disk verification and repair capabilities. These applications cannot fix the directory structure on the startup disk, so starting up the computer from the CD or from another disk.

3. DiskWarrior is sold by Alsoft; this application is specifically designed to examine and repair the contents of a disk's directory information. To use it, open the application, choose the disk from the window which appears, and click the Rebuild button. In some cases, the computer will need to be started up from the DiskWarrior CD to repair the disk.

4. In a case where only data needs to be recovered without there being any problems with the directory structure, or if repairing the directory structure isn't needed, applications such as Data Rescue can be used for this purpose.