There are few worse error messages to see on your Mac than this one:
Error: Disk Utility can’t repair this disk. Back up as many of your files as possible, reformat the disk, and restore your backed-up files.
It could appear when you’re using Disk Utility in a bid to try and fix a problem, and Disk Utility has simply shrugged its shoulders and given up.
Perhaps surprisingly, all is not necessarily lost. You might indeed need to wipe and reinstall but before that you can at least try a few other tricks.
Before attempting anything, however, you should backup your disk contents because it’s clear there’s a potentially huge issue and you’re living on borrowed time. If you haven’t already got one, beg, borrow or steal an external hard disk of sufficient size and attach it by USB, FireWire, Thunderbolt or Wi-Fi. Then use Time Machine, the OS X built-in backup software. Note that Time Machine only works with a handful of NAS – check the manual.
When the disk is attached Time Machine should detect it but if not open System Preferences, click Time Machine, and move the large switch to the On position. Then follow the instructions.
If you don’t fancy using Time Machine, apps like Carbon Copy Cloner will do a similarly good job.
Attempting a last-ditch repair
Reboot your Mac and, before the Apple logo appears, hold down Cmd+S. Release when the boot progress indicator appears. This will boot you into Single User mode and things will suddenly go very Linux, with a text-based display, as shown above. Don’t panic! Just wait until the flashing text cursor appears and type the following:
If you see the following message at the end of the repair:
***** FILE SYSTEM WAS MODIFIED *****
… then type the command again. Repeat another two or three times until you have a message similar to this returned:
** The volume appears to be OK
Of course, you may never see this because even this trick might not be able to fix your disk. However, you might be able to make note of its output and google it to see if you can find a solution.
When you’ve finished in Single User Mode, type the following to reboot:
Note that you might not be able to enter Single User Mode if you have a firmware password set. You’ll need to temporarily disable it. Additionally, on a Retina display the text will be tiny – but still readable if you move closer to the screen!
Commercial rescue apps
You might want to try some of the commercial disk repairing apps like TechTool Pro but the quickest solution might be to do as Disk Utility suggests, which is to wipe the disk, and restore from a backup – or wipe the disk and reinstall OS X. Both options are available within OS X Recovery, accessible by holding down Cmd+R while booting your Mac.