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I am trying to erase the hard drive of my MacBookPro and perform a fresh install of Mac OS X Mountain Lion. It already has the said os installed. I have a Mountain Lion installation USB key handy. When I go through the erase steps in the Disk Utility, the erase button remains grey, so I cannot erase partition containing the already installed OS.

I go into Disk Utility, select the hard drive I want, click erase, but the format window (Mac OS Extended) remains grey. The name bar is untitled, and there is no option available change it. The erase button at the bottom left remains grey, I cannot erase the partition.

How do I resolve the issue?

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You have to boot off another drive before you can erase your internal hard drive.

Boot off your external USB drive and then you should be able to erase the internal one.

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You should boot from the recovery partition, to format the internal hard drive.

To boot from the recovery partition, hold the option key immediately after you turn the computer on. You will see two drive icons on the screen ... one for your internal hard drive, and the other for the recovery partition. Choose the recovery partition, and from there, you can run Disk Utility.

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