by Your Mac has begun showing signs of trouble. Perhaps you frequently get errors when trying to open or save files. You suspect a problem with the hard drive. Before panic sets in, you want to launch Apple’s Disk Utility and select Repair Disk from the First Aid tab. Hopefully, that will remedy the situation. One problem though: Repair Disk is dimmed and you can’t select it. Why? Because OS X cannot attempt repairs on an active startup drive. You can still use Repair Permissions, which may help in certain situations. But let’s assume it doesn’t.
So what do you do instead? That depends on what Macs you own, how you have set them up, and what other precautions you may have taken prior to the start of the trouble.
First things first, if you don’t have a recent backup, make one now. But be careful. At this point, you don’t want to overwrite an existing backup—lest you replace valid data with corrupted data. Instead, back up to a separate drive. When you’re done backing up, here are the things to try. You can try each method until you find one that works:
Boot from the startup drive’s Recovery HD partition
The startup drives of Macs formatted with OS X 10.7 (Lion) or 10.8 (Mountain Lion) typically have a hidden partition designed just for moments like this. This 650MB partition is called Recovery HD. Boot your Mac from Recovery HD by holding down Command-R at startup (or by choosing it from within Startup Manager, which you access by holding down Option at startup).
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