Your hard drive failing can mean loss or corruption to important data, or perhaps to applications and system software that can destabilize the system. If your Mac is regularly showing hangs, is running exceptionally sluggishly, crashes, or you get odd permissions denied errors about the inability to access certain files for which you previously had access, then your drive may be about to fail.
Before any detail of hard drive health is considered, be sure you have a full backup of your drive. Therefore, even if your drive is not experiencing problems, set your system up with Time Machine or a system cloning tool, and get a regular backup going that can be used to restore your system to a fully-working state.
The first option for testing your drive is to check its S.M.A.R.T. status, which is a series of built-in tests that the drive regularly checks on itself. If any of these are out of place, then the drive will notify the system when a S.M.A.R.T. verification is performed. This can be done at any time by opening Disk Utility (in the Applications > Utilities folder), selecting your drive device, and reading the S.M.A.R.T. status at the bottom of the window. If this status says anything but “Verified,” then you need to replace your drive.
Select the drive device (above any volume names) and then check the S.M.A.R.T. status at the bottom of the window (click image for larger view).
Disk Utility is not the only option for checking the S.M.A.R.T. status, as there are numerous third-party programs like S.M.A.R.T. Utility (some of which are free), that may be an even more thorough S.M.A.R.T. verifier than Disk Utility.
If the S.M.A.R.T. status shows no problems, then check the disk’s formatting with Disk Utility, and do this regularly. With use, the formatting on your drive can become damaged, but if your drive is experiencing problems then format damage may occur at a higher rate. If your first check shows any formatting errors, then boot to the Recovery HD partition by holding Command-R at startup, and use Disk Utility there to fix the drive. Follow this by checking the drive formatting regularly (every other day, or more), to ensure no more errors crop up. If they do, then this indicates the drive may be failing.
Finally, use a third-party utility like Drive Genius or Disk Tools Pro to check the drive’s media integrity with a surface scan. This will check for bad blocks, and replace them with spare ones, if necessary. If you do find bad blocks with a scan, then again repeat this scan the following day after using your system for a while, and continue to do so for a few more days. If bad blocks regularly appear, then this suggests the drive will likely need to be replaced.
For external drives, unfortunately S.M.A.R.T. status checking is not supported; however, you can still check their formatting and perform surface scans. In addition, you can troubleshoot any daisy-chain setups you have to ensure proper connectivity and power for drives. Improper daisy-chaining can lead to loss in power or data connection that can corrupt drive contents, causing it to either not mount, or suddenly eject.
Lastly, for any mechanical drive, be it internal or external, if you cannot mount it, then hold your ear to the drive and see if you can detect any regular clicking, whirring, or grinding sounds at regular intervals. Most drive noise should be random or continuous, and regular intermittent clicking indicates the drive is attempting to perform some action, failing, and then repeatedly trying again.
If this is the case, then you can try the following:
- Check power connections for the drive and troubleshoot daisy-chaining of it and other peripherals.
- Dissect the device and place the raw drive in a new enclosure, since a failing USB or FireWire bridge controller in the enclosure, or fault in power handling, may result in the drive failing.
- Attempt a fix for deep formatting problems with the drive.
- Replace the drive.