Unusual behavior on your Mac may have a number of causes, but often the source is a failing hard drive. One place to check for issues is the OS X System Log, which can tell you whether there's an I/O error that might indicate a disk issue. By using some of your Mac's other system utilities, you may find the cause of the problem before your disk goes down completely.
If you're noticing sudden problems such as temporary system freezes, or files and applications that fail to open, check your System Log to see if it displays an error message. To view your System Log, go to the "Applications" folder. In the Utilities folder, select the Console app. The System Log lists the recent errors OS X has encountered. An I/O error message usually follows0o9r5 this pattern:
Jan 22 10:21:13 yourusername kernel: disk0s2: I/O error
The name "disk0s2" indicates the volume name. If your disk is partitioned, it's the partition where the error occurred.
Repairing the Disk
Mac OS X also includes Disk Utility, which enables you to test and repair a problem disk. When you open Disk Utility, click the name of the disk itself, usually listed as the brand and size of the disk. If the disk has the "S.M.A.R.T" self-diagnosis feature, look at the disk information at the bottom of the window. If the status is "Failing," back up the disk and replace it. To verify the disk, click "First Aid" and click "Verify Disk." If the disk contains your OS X System folder, you won't be able to repair the entire disk unless you first boot from a System CD. If Disk Utility returns an error on a non-system partition, select the partition and click "Repair Disk." If the problem is on your system partition, boot your Mac from the system CD, open the copy of Disk Utility from the CD and repair the partition. Some problems can be solved using third-party software such as Alsoft DiskWarrior or MicroMat TechTool Pro.
Backup and Replace
If you can't repair the problem and haven't recently backed up your data, back up immediately to another drive. To back up the drive using Disk Utility, drag the partition from the left side of the window into the "Source" box, then drag the disk you want to copy the partition to into the "Destination" box. To replace your drive, find your model's instructions on removing and installing a hard drive. Apple uses standard disk sizes, and you can format your new disk by booting with the System CD, opening Disk Tools, clicking "Partition" if you want more than one partition on the drive, or clicking "Erase" to format a single partition.
Hard disk failure is a common problem. Prepare for it with the expectation that your disk may fail at any time. The easiest method for maintaining backups is with Apple's Time Machine, a feature of OS X. Time Machine makes incremental backups to an attached external drive, and when you must replace the disk, you can copy everything back from your Time Machine backup, including your system settings and preferences. Check your disk periodically with Disk Tools or a third-party utility to fix small problems and get early warnings of big ones.
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