Erase and Reinstall OSX

Erase your Mac, then reinstall OS X using the built-in recovery disk. Before you erase, back up your essential files. If youre installing on a portable computer, make sure your power adapter is plugged in.

Important: To reinstall OS X, you must be connected to the Internet.

1. Choose Apple menu > Restart. Once your Mac restarts (and the gray screen appears), hold down the Command (?) and R keys.
2. Once you see a desktop, verify you are connected to the network (usually via WiFi) If youre not connected to the Internet, choose a network from the Wi-Fi menu (in the top-right corner of the screen).
3. Select Disk Utility, then click Continue.
4. Select your startup disk on the left, then click the Erase tab.
5. Choose Mac OS Extended (Journaled) from the Format menu, enter a name, then click Erase.
6. After the disk is erased, choose Disk Utility > Quit Disk Utility.
7. Select Reinstall OS X, click Continue, then follow the onscreen instructions.

Hopefully that worked but its possible your Built-in Recovery Disk may not work.  Let's walk through, "The Recovery Disk Assistant"

The Recovery Disk Assistant lets you create a Recovery System on an external drive that has all of the same capabilities as the built-in Recovery System.

Recovery Disk Assistant is provided for use with Macs using OS X Lion or OS X Mountain Lion.

With Recovery Disk Assistant, you can create a Recovery System on an external drive that allows you to reinstall OS X, repair the disk using Disk Utility, restore from a Time Machine backup, or browse the web with Safari. This drive can be used in the event you cannot start your computer with the built-in Recovery System, or you have replaced the hard drive with a new one that does not have OS X installed.

System requirements

• A Mac using OS X Lion or Mountain Lion with an existing Recovery System on its startup volume.
• An external USB hard drive or thumb drive with at least 1GB of free space

If you have a newer Mac, you can use Internet Recovery to start up from an Internet-based version of the Recovery System. This can be more convenient than using this utility.

You can also install create a bootable OS X installer in OS X Mavericks or OS X Yosemite.

How to use Recovery Disk Assistant

The Recovery Disk Assistant erases all data on the external drive you select when creating the Recovery Disk. You should either backup your data before running the Recovery Disk Assistant, or create a new partition on your external drive.

If you need to create a new partition on the external disk

1. Open Disk Utility, located in the Utilities folder in Launchpad.
2. Select the drive on which you would like to install the Recovery System and add a partition. The partition needs to be at least 1 GB in size.
3. Click Options and make sure GUID Partition Table is selected.
4. Make sure the format for the partition is Mac OS Extended (Journaled).
5. Click Apply.

Open Recovery Disk Assistant and follow the on screen instructions to create a Recovery System on the external drive.

When the Recovery Disk Assistant completes, the new partition will not be visible in the Finder or Disk Utility. To access the external Recovery Disk, connect the drive, then restart the computer and hold the Option key. Select your external drive from Startup Manager.

Notes

• If your computer came with OS X Lion or OS X Mountain Lion, the external recovery drive you create with this utility should only be used with the computer that created it. Other Mac models may not be able to start from this recovery drive.
• If your system was upgraded using the OS X Lion or OS X Mountain Lion installer purchased from the App Store, the external recovery drive can be used with other similarly-upgraded systems.

Built right into OS X, OS X Recovery lets you repair disks or reinstall OS X without the need for a physical disc.

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The OS X Recovery Disk Assistant lets you create OS X Recovery on an external drive that has all of the same capabilities as the built-in OS X Recovery: reinstall Lion or Mountain Lion, repair the disk using Disk Utility, restore from a Time Machine backup, or browse the web with Safari.

Note: In order to create an external OS X  Recovery using the OS X Recovery Assistant, the Mac must have an existing Recovery HD.

To create an external OS X Recovery, download the OS X Recovery Disk Assistant application. Insert an external drive, launch the OS X  Recovery Disk Assistant,  select the drive where you would like to install, and follow the on screen instructions.

When the OS X Recovery Disk Assistant completes, the new partition will not be visible in the Finder or Disk Utility. To access OS X Recovery, reboot the computer while holding the Option key. Select Recovery HD from the Startup Manager.