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I'm trying to do a clean install of Mountain Lion but I'm not being able to format my partition.

I previously had Lion, encrypted the disk, then upgraded to Mountain Lion, then booted into recovery mode to format & reinstall Mountain Lion.

On recovery mode (Cmd+R), when I tried to format the encrypted partition I got an error saying something like "not enough space on core something". Now the partition doesn't show up on Disk Utility; only the disk, "Macintosh HD", but I am not able to repartition it.

The Mountain Lion installer doesn't find any disk to install itself on.

I cannot boot the system either (without booting to recovery).

If I open the terminal and type diskutil list I can see that there's an Apple_CoreStorage on /dev/disk0s2, but I cannot reformat that volume ("The disk is in use by Core Storage as a Physical Volume").

How to format the disk and install Mountain Lion?

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4 Answers 4

up vote 1 down vote accepted

You should be able to use dd to destroy the partition's header. You can then format (erase) it and run the install.

From the installer run Terminal and type the following:

dd if=/dev/zero of=/dev/disk0s2 bs=1024 count=1024

Do be warned, this will irrecoverably destroy all data on that partition. But it looks like that's what you're expecting. Make sure you don't accidentally target the wrong thing.

Also note that I haven't done this, exactly. I've used this technique to destroy the entire partition table, not just a single partition. But I think it should work.

When you're done quit Terminal and run Disk Utility. You won't get any complaints about Core Storage.

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This did not work, I still have the exact same issues. –  lzm Aug 2 '12 at 6:23
    
Is it safe to format the entire /dev/disk0? The volumes EFI and Apple_Boot seem to be important. –  lzm Aug 2 '12 at 6:25
    
That will also destroy your recovery partition. If you have an external disk to boot from (or a newer Mac that will netboot over the Internet) then yes it's safe. You just need to re-partition afterward. –  bahamat Aug 2 '12 at 6:27
    
Ok, do you have more information about how to recreate those partitions? –  lzm Aug 2 '12 at 6:33
4  
Creating a single partition with Disk Utility will restore everything except the recovery partition. If I understand correctly, the OS install will create the recovery partition. –  bahamat Aug 2 '12 at 6:35

When the Partition tab of Disk Utility in Mountain Lion can not manage a partition on a physical disk that uses Apple Core Storage:

Ignore the Partition tab

Instead of selecting the logical volume group (to the left):

  • select the encrypted logical partition (to the right of, and below, the LVG)
  • use the Erase tab.

What techniques for disk, volume and file system management are possible with OS X, but troublesome within Disk Utility?

In particular:

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Without destroying all partitions/slices of the physical disk:

Use the coreStorage verb of diskutil

To delete the logical volume group that is no longer required:

  1. get the UUID of the group
  2. run a command based on the following format:

diskutil coreStorage delete lvgUUID

If a logical volume within the group is mounted, it may be necessary to unmount before attempting destruction. Also maybe necessary to use sudo … sorry I can't be more specific, it's a couple of weeks or so since I last performed an action of this type .

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I'll have to try this. –  bahamat Aug 2 '12 at 17:20

Check out this solution:

OS X Mountain Lion “clean” install gotcha: CoreStorage / Encrypted Disk issue

Basically, you need to run diskutil CoreStorage list from Terminal. Identify the logical CoreStorage volume from that list and note the UUID string.

Then type diskutil CoreStorage delete UUID where UUID is the string you identified.

You can then partition with Disk Utility and run a Mountain Lion install.

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