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If you see a prohibitory symbol at a startup disk, installed an older version of the OS X inadvertently or if you have erased the startup disk, you might consider reinstalling OS X on your Mac. Even when you are setting up a new startup disk, you can reinstall OS X, which can be done either by reinstalling from recovery, restoring OS from a backup or by erasing a drive and then installing the OS.
The following are three methods to reinstall OS X Mac. You can use any one of them according to your convenience.
Reinstall OS X on your Mac from recovery
- Power on your Mac
- When you hear the startup sound, hold the Command and R keys on the keyboard
- Release the keys once the Apple logo appears on the screen
- You will get prompts to select a Wi-Fi network, or connect Mac to the internet using Ethernet cable
- Select Install OS X option from the recovery menu
- Follow the on-screen prompts to select the disk and install OS X
Restore OS X from backup
If you have created a backup of your system using Time Machine, then you can restore from the backup to reinstall OS X.
- Restart your Mac. When the startup sound comes, hold the Command and R keys to enter into recovery
- From the recovery windows select Restore from Time Machine Backup
- Select Time Machine backup disk then select the Time Machine backup you want to restore
- Click Continue and follow the onscreen instructions to reinstall OS X
Erase your drive and install OS X
- Connect your Mac to a stable internet connection
- Restart your Mac. When you hear the startup sound, hold the Command and R keys to put the Mac in recovery
- From the Recovery window select Disk Utility then click Continue
- Select the volume name of your disk from the left side of Disk Utility window then click Erase
- Click Security Options if you are looking for methods to securely erase the drive. Select an erase method and click OK
- Now from the Format pop-up menu, select Mac OS Extended (Journaled). Type a name for your disk then click Erase
- When the erasing process is over, close the Disk Utility tab
- Select Reinstall OS X, click Continue and then follow the onscreen instructions to reinstall OS X
Tip: When you want to reinstall a version of OS X that shipped with your Mac, or when you want to reformat the startup disk entirely, just hold Command option and R at startup. Doing so starts your Mac from internet recovery. In case you are having more than one partition and want to erase the startup disk, use Disk Utility to remove partitions or, repartition your drive.