Erase and reinstall OS X mountain lion

Question

Level 1

8 points

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

Answer 1

babowa

Level 9

78,852 points

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

Answer 2

Ri-dog

Level 1

45 points

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

Answer 3

babowa

Level 9

78,852 points

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.

Answer 4

babowa

Level 9

78,852 points

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.

Answer 5

willp240 Author

Level 1

8 points

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.

Answer 6

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

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

Answer 7

willp240 Author

Level 1

8 points

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.