Verify and repair startup disk on OS X

Repairing Disks using OS X's Disk Utility can be useful in a variety of scenarios and can result in improved performance, and in some situations - can recover lost data.

The most obvious example would be disk corruption caused by improperly removing external USB storage. It's not guaranteed to recover lost data however.

It can also be used on your internal partition, if you are having trouble with OS X, but requires the computer be booted in Recovery mode to access this. As repairing the internal partition where OS X is installed, can have serious repercussions, we advise performing a back-up of any crucial data before proceeding. 

Repairing a disk

You can locate Disk Utility via Macintosh HD --> Applications --> Utilities, or via  a search for "Disk Utility" in Spotlight.

Once you've loaded Disk Utility, select 'Verify Disk' and allow your Mac to check the disk. It should give you some diagnosis to confirm whether it's viable, or needed to repair.

If using an external disk, then choose 'Repair Disk', and allow the process to complete.

If trying to repair the internal partition you've got your OS X installation on, you'll need to boot into Recovery mode, so your Mac can check and repair files that are "in use".

Restart your Mac, and press Command + R, while it's restarting. Select Disk Utility from the OS X Utilities menu. Once Disk Utility has loaded, choose the disk you wish to repair - the default name for your system partition is generally "Macintosh HD", and choose 'Repair Disk'.