Fix your Apple Mac's disk with a secret power tool

The disk in an Apple Mac is probably the one component that is most likely to fail. Perhaps that is because it is neither designed nor built by Apple. There is a geeky secret tool that can be used to repair faults and here's how to use it.

Disk storage is amazingly reliable and drives often work for years or even a decade or more without failing, but that is just the hardware. Most disk problems have little to do with the actual hardware. The mechanical components or solid state storage chips rarely develop faults.

Most disk problems are software-related. This means that files are corrupted or even the filing system itself becomes damaged and OS X (macOS) can't locate files or parts are missing, and so on.

Small disk problems seem to crop up from time to time just through normal Mac use. They don't usually cause any problems, but if these minor problems are not fixed then they can become worse and sooner or later you will have a major disk problem.

Related: 3 ways to fix disk errors and repair permissions
Related: Get to know the new Disk Utility in El Capitan

Easy disk fixes

Disk errors can be avoided to a large extent by regularly checking the disk drive for faults - and fixing them. One way to do this is by running Disk Utility in the Applications\Utilities folder.

Select the Mac volume on the disk drive (not Bootcamp if you have it), and then click the First Aid button in the toolbar. Click Run in the window that pops up.

This is easy and it is fine for simple disk errors. However, Disk Utility cannot repair all disk problems and so a more powerful tool is required. The tool you need is on the disk, but it is unusual and it isn't something you will discover by accident.

Boot up in single user mode

Single user mode is a special startup mode that is used for finding and fixing various problems with the Mac. It can be used to repair disk faults.

Press the power button and as soon as you hear the startup sound or the screen turns gray, press and hold down Command+S. The screen fills with text and the end result is a Mac that looks something like this:

There are no icons, no desktop image, no menus... nothing but text. Don't worry, your Mac is fine.

Check the disk and fix faults

At the command prompt, type fsck -fy

The first line highlighted above shows fsck -fy being entered at the command prompt. Various messages are displayed and the last one says The volume MacBook appears to be OK. (My disk is named MacBook, but your disk might have a different name.)

If problems are found and messages displayed, run fsck -fy again and again until it says that the disk is OK.

Return to normal

So how do you get out of single user mode? Tyep exit to quit and boot to the desktop via the login screen. Don't worry about the text messages that appear, they will disappear the next time you reboot. You can also reboot by typing reboot. The Mac will then start as normal.

Further reading

If you are wondering what fsck means, it stands for file system check. The -f fixes the disk and the y answers Yes to every question. You could leave out the Y and when errors are found you'll be asked if you want to fix them, but it is usually best to add the y switch and fix them all automatically.

For more information on fsck see the Apple OS X manual page.