How to fix a feeble hard drive

Sooner or later, nearly every Mac user encounters a hard disk problem. Because the hard drive stores the data that enables your Mac to run, such problems can cause your Mac to misbehave seriously-applications can quit without warning, your Mac might get stuck during startup, files might become corrupted.

Fortunately, if you think you might have a hard drive problem, there are several easy steps you can take to diagnose and (often) repair it.

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Many hard-drive malfunctions are the result of logical problems (such as errors in particular files on your disk or in the hidden directory structures that keep track of them). Symptoms can range from applications that fail to launch or display error messages to extremely slow response from your Mac. To troubleshoot logical problems such as these, try the following:

Free up disk space. When your disk is close to being full, performance can suffer dramatically. In the Finder, select your disk and choose File: Get Info. If the amount listed next to Available is less than 10 GB or so, you should immediately delete files (or move them to another drive or an optical disc) to give OS X more breathing room.

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Repair disk permissions. Some people recommend that you run Disk Utility’s Repair Disk Permissions tool regularly, as a kind of preventive maintenance. I don’t. But I do think that repairing disk permissions can help when you’re troubleshooting a misbehaving disk; there’s certainly no harm in trying it. To do so, open Disk Utility (in /Applications/Utilities), select a disk or volume in the list on the left, click First Aid, and then click Repair Disk Permissions.

Run a disk-repair program. Several utilities can check for and repair directory damage, file corruption, and other common logical errors. Apple’s Disk Utility is one of them, but others-including Alsoft’s $100 DiskWarrior , Prosoft Engineering’s $99 Drive Genius 2, or Micromat’s $98 TechTool Pro -offer more-advanced features.

I usually try Disk Utility’s Repair Disk feature first; if that doesn’t work, I move on to one of the others. Regardless of which program you use, you should start up from another volume (such as your Mac OS X Install DVD, a startup CD or DVD included with third-party repair software, or a bootable duplicate stored on another hard drive) and run the software from there; most disk repairs can’t be performed on the disk from which the software is running.

If none of the steps above solve your problem-or if you can’t perform them because your disk won’t even mount on the desktop-you should check for a hardware problem. To do so, try the following:

Check the cables. It can be embarrassing, but it’s happened to me numerous times: I thought an external drive had died, only to discover that a cable was loose or that the cat had knocked the plug out of the wall. Check your drive’s data cables (USB, FireWire, or eSATA) and AC adapter (if any) on both ends to make sure all connections are solid.

Try another port. If the cables are OK, check the ports they plug into. If your Mac has another port of the same kind (a second USB, for example), move the cable over to it. If the drive has more than one interface (USB and FireWire, say), try connecting the drive with that other interface. Or, if you have a second Mac handy, try connecting the drive to it to see if that solves the problem.

Check the power supply. If your drive’s power light doesn’t come on (or if it’s dimmer than usual), if the drive makes a whining sound, or if it starts to spin up and then immediately stops, the problem could be a faulty power supply. I’ve had two AC power supplies for one LaCie drive go bad, even though the drive itself (and the data on it) was fine.

If you suspect a problem with your drive’s power supply, contact the manufacturer. If the drive is still under warranty, the company may replace its power supply at no cost. If the warranty has expired, the company may sell you a new drive at a discount. Another option for out-of-warranty drives: Open the case, remove the drive mechanism, and move it to a different enclosure.

Recover your data. If none of these steps help, and you don’t have a backup, your last resort may be to recover whatever information you can and then replace the drive. If the drive is mountable, Prosoft Engineering’s $99 Data Rescue 3 may be able to retrieve data from it. If that doesn’t work, you may need to enlist the services of a data recovery service such as DriveSavers. Keep in mind that such services are expensive and should therefore be used only as a last resort.