Erase and reinstall OS X mountain lion

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Erase and reinstall OS X mountain lion

I already have mountain lion installed on my iMac. However, due to problems and overall frustration, I want to erase everything and reinstall. I did the restart, cmd r, and opened the recovery. I downloaded ML again, which restarted the computer. I then had to restart again to get back not the recovery. I go nit the disk utility to erase what's there and reinstall ML, but it won't erase because it says it cant unmount. What am I doing wrong?

MacBook Pro, Mac OS X (10.7.3)

Posted on Jan 31, 2013 4:37 PM

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Answer 1

Jan 31, 2013 5:35 PM in response to willp240

You have to boot from the recovery drive to erase and install.

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Answer 2

babowa

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Jan 31, 2013 5:40 PM in response to willp240

You can't erase the drive you're booted from. You need to go into recovery (Command + R or Command + Option + R) while restarting, then go into Disk Utility. At that point, you are booted from and connected to Apple's servers. You can then erase. When that is done, you can download and install.

I believe what you did is download and install and then tried to erase, which will not work.

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Answer 3

willp240 Author

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Jan 31, 2013 5:58 PM in response to babowa

But I have booted from the recovery but hitting command r while booting up and done all of this. When I go into recovery from boot up, I go to erase and says it cannot unmount the image.

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Answer 4

babowa

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Jan 31, 2013 6:04 PM in response to willp240

You need to use Command + Option + R (that is Internet Recovery, which is a bit different) which will get you to be booted from Apple's servers; I've not heard of an instance where someone could not erase the drive from there.

Read about Internet Recovery here:

http://support.apple.com/kb/HT4718

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Answer 5

willp240 Author

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Jan 31, 2013 7:32 PM in response to babowa

Ok, I did the Internet recovery, but the problem persists. I go to erase, and it says "volume erase failed with the error: couldn't unmount disk."

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Answer 6

Barney-15E

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Jan 31, 2013 7:40 PM in response to willp240

Are you selecting the Macintosh HD or the whole disk?

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Answer 7

willp240 Author

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Jan 31, 2013 7:42 PM in response to Barney-15E

I tried both and they both gave the same error message

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Answer 8

Jan 31, 2013 7:47 PM in response to willp240

What is your hardware and OS?

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Answer 9

willp240 Author

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Jan 31, 2013 7:51 PM in response to Frank Caggiano

OS X 10.8.2, 2.7 ghz intel, 4 bg memory

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Answer 10

Jan 31, 2013 7:53 PM in response to willp240

What about the system? which iMac?

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Answer 11

willp240 Author

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Jan 31, 2013 7:55 PM in response to Frank Caggiano

It's the 21.5imac.... Mid 2011. OS X 10.8 (mountain lion)

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Answer 12

babowa

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Jan 31, 2013 8:15 PM in response to willp240

Since you've already installed the OS once, unfortunately, I don't know what to suggest next - I've not come across your situation before. Unless someone comes up with a brilliant idea, my suggestion would be to book an appointment at your nearest Genius Bar and show them the problem for a diagnosis/fix.

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Answer 13

Ri-dog

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Jan 31, 2013 8:31 PM in response to willp240

I would suggest installing the OS X Recovery Disk Assistant onto an external thumb drive: http://support.apple.com/kb/DL1433, and then booting from that (holding down option key at startup and selecting the external drive). Try erasing Macintosh HD from there and then reintalling.

Regards,

Ri-dog

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Answer 14

babowa

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Jan 31, 2013 8:48 PM in response to Ri-dog

That's a good idea - hopefully that will work!

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Answer 15

willp240 Author

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Feb 3, 2013 4:45 PM in response to babowa

I have follwed these instructions exactly. I chose the external harddrive with the disc assitant installed on it. The utilities screen popped up, I selected the disk utility, went to "Macintosh HD," went to erase, and it still says that the image cannot be unmounted.

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