Knowing that OS X is quite large at approximately 6 GBs, it can take some time, even on a fast Internet connection. If you are using a recent model Mac, this is dependent on having a fast Internet connection. If Disk Utilities does not work, then I would look at using the OS X Reinstall option. In the sidebar select your OS X disk click the First Aid tab click Repair Disk If you are running an earlier version of OS X, you can still give it a try. If you are running OS X El Capitan (10.11) or later this option no longer applies since Apple now protects file system permissions automatically. Be careful not to click anything named partition or erase if you have data you would like recover. You can use this to perform basic operations such as repairing disk permissions and check the health of your system. The first one I would start off with which is not too invasive is Disk Utilities. The OS X Utilities screen presents a list of typical recovery options you can try to get your Mac up and running. Select your language then click Continue. After loading, you will be greeted by the welcome screen below. Exercise a little patience here as the recovery options seem to load a mini copy of OS X so it will take a little while. To do so, shut down your Mac then start it, immediately before the Apple logo appears on screen, press Command + R keys. If you are unable to start your Mac successfully and not get to the login screen or even load the Finder, the first option is to turn to your recovery utilities that are built-in.

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