Disk Utility (Yosemite): Check a disk for problems
You can use Disk Utility to check a disk for problems, then repair them. While Disk Utility is checking your disk, your computer may become slower.
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Select a disk or partition in the sidebar, then click First Aid.
If you run First Aid on a disk, Disk Utility checks the partition maps on the disk and performs some additional checks. If you run First Aid on a partition, Disk Utility verifies all the contents of that partition.
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If Disk Utility tells you the disk or partition is about to fail, back it up and replace it. You can’t repair it.
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Click Verify Disk.
If Disk Utility reports that the disk appears to be OK, you’re done. Otherwise, you need to repair the disk.
When checking your disk for problems, you can also view the disk activity log by clicking Log in the Disk Utility toolbar.