Section 145. Verify and Repair a Disk

145. Verify and Repair a Disk

BEFORE YOU BEGIN

21 Add a Newly Installed Hard Disk to the System

143 Back Up Your Information

SEE ALSO

146 Restore or Duplicate a Disk

Hard disks have a bad habit of deteriorating over timeperhaps more so than any other component in a computer. This is especially problematic in that your hard disk isn't just a component that you can replace; it's the permanent memory for your computer, the place where all your accumulated data and important documents are stored. If you haven't done a backup (and why haven't you?), and your hard disk goes bad, you will likely be willing to do just about anything to get that data back.

The Disk Utility application, located in the Utilities folder inside Applications , provides a handy way to perform quick repair operations on your hard diskoperations that have a surprisingly high success rate in many cases. Disk Utility can examine the Unix permissions on your hard disk and detect discrepancies that might cause instabilities in your system; it can also correct those errors for you. What's more, Disk Utility can scan your disk for surface defects (the most common cause of data loss from deterioration over time), and even "repair" these errors by recovering the data in the defective regions and copying it to a healthy part of the disk.

NOTE

You must be logged in as an Admin user to verify or repair a hard disk.

145. Verify and Repair a Disk

1.	Boot from a Different Disk Determine whether the disk you want to inspect is your Mac's startup disk. If it is, you won't be able to do repairs on it while the Mac is booted from it. Boot the system from an external backup disk or from the Mac OS X installation CD to do repairs on your main startup disk. Another method is to use a second Mac, by booting your computer into FireWire Target Disk Mode and connecting it to another computer using a FireWire cable. You can then run Disk Utility on that second computer, operating on your main Mac's hard disk. See 141 Boot from Different Disks Using Keystrokes .
2.	Open Disk Utility Navigate into the Utilities folder inside the Applications folder. Double-click the Disk Utility icon to launch the application.
3.	Select the Disk to Verify From the list in the sidebar of the Disk Utility window, select the disk you want to examine for errors. You can select an entire disk or a single volume within that disk. Click the First Aid tab to access the disk repair functions.
4.	Verify and Repair the Disk Permissions Click Verify Disk Permissions to begin a scan of the permissions of all the files on the disk. This process will take several minutes; Disk Utility reports all its findings in the status window. TIP If you notice results in the status window that indicate a possible reason for instability in the system, repair the permissions by clicking Repair Disk Permissions . This process, too, can take some time. When the repair is complete, however, your Mac OS X system and its applications might behave more predictably if you've been seeing a permissions- related problem.
5.	Verify and Repair the Disk To perform a surface scan of the disk, click Verify Disk . This process can take several minutes or up to an hour or more. The results of the process are shown in the status window. NOTE To do a surface scan of the disk, Disk Utility must be able to unmount it from the system. Make sure that you are not using any applications or documents on that disk, or Disk Utility will report an error and fail to perform the verification. If Disk Utility reports correctable errors in the status window, and you want to recover your disk and make it usable again, click Repair Disk. Disk Utility will again unmount the disk and attempt to repair any surface inconsistencies that it detected during verification. When Disk Utility has finished the repair process, restart your computer using the regular startup disk (if necessary); with any luck at all, your disk will be back in working order. However, even if you were able to resurrect your disk using Disk Utility , be aware that any surface defect that causes you to do a repair is an indication of an aging disk that should be replaced . Consider buying a new hard disk for your computer, and by all means back up your data using the .Mac Backup application as described in 143 Back Up Your Information . At the least, copy your files to CDs or DVDs. You'll thank yourself later!