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

As I mentioned earlier, there appears to be an issue with the Disk Utility that comes bundled with the Mountain Lion recovery/installation system.

If you’re looking to do a completely clean Mountain Lion install, and already have an encrypted filesystem created by FileVault from OS X Lion – you may encounter the following problem when attempting to erase or delete it through Disk Utility. I have been able to reproduce this across two machines now (an early 2011 17″ MacBook Pro and a mid-2011 21″ iMac).

Once you’ve booted from the USB drive and fired up Disk Utility – if you delete the encrypted volume, you’ll find you’ll come across this message:


You won’t be able to do anything at all with the boot volume – you can’t create a new partition, you can’t install – you’re snookered.

But don’t panic! Just quit Disk Utility, go to the Utilities menu and fire up Terminal. Then issue the command:

diskutil cs list

which will give you a list of logical CoreStorage volumes (and a physical volume as well – but ignore that). Copy the long UUID string of the the logical volume. Now type:

diskutil cs delete UUID

where UUID is that long string of characters. You will see a text-based progress bar for a bit and finally confirmation that the volume has been deleted. The whole process looks like this:


You can then quit Terminal, fire up Disk Utility, partition to your heart’s content and then finally install a fresh clean Mountain Lion from scratch. Hoorah.

  • Colin

    Awesome. I ran into a couple of gotchas while trying to perform a clean install of Mountain Lion.

    First related to creating the USB installer with Disk Utility and I found a fix here:

    Second gotcha was exactly as described here because of my previously Lion encrypted drive. Your fix saved me a lot of time. Thanks.

    • Martyn

      The only issue I had initially when creating the USB image was that if I didn’t mount InstallESD.dmg and restore from that (so rather than dragging the InstallESD.dmg to Source in Disk Utility’s Restore function, I instead dragged the contents of the mounted InstallESD.dmg image instead), I’d get an unrecoverable error at the end of the imaging process.

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  • tsr

    Thanks for the post. I had the same problem when I went to do a clean install. Your solution worked.

  • Patrick

    Thank you! Your post saved my day and I should really get in touch with the terminal utilities under Mac OS ;-).

  • http://www.g8production.com Daniele Galiotto

    thank thank thank thank youuuu!!!!

  • Anderson

    Holly s… U really rock! Tx a lot!

  • http://www.kankei.fr Jean-Philippe

    You saved my night!
    Awesome! Thank you so much!

  • Bertrand

    Thank you SO much !
    Been stuck for two days with that, i feel better now !
    Thanks to you ;)

  • Scott Anderson

    Thank you very much. New to macs (and struggling after years of windows)
    Trying to reinstall after multiple issues with macbook air and fell at the first hurdle.
    Apple support response was useless – they were confused!
    Found your post, followed the instructions and Bingo – perfect

    Thank you again


  • Michel Forgues

    I just used this little trick to erase a TimeMachine partition that wouldn’t accept the encryption password anymore. Yes, I had the right password, had used it before and stored it in KeyChain and 1Password. The other (non-encrypted) partition on the same physical disk was left untouched and unharmed. The only damage seems to be the loss of my TimeMachine backups, a lesser problem since I have multiple non-TM backups as well.

    What is the point of FileVaulting your startup disk if the backup is NOT encrypted as well?

    Now, why should this happen in the first place?

    So, a million thanks to people smarter and more knowledgeable than myself!

  • Nasser

    Thank you Very Much

  • John

    Drake: you are a super hero. You’ve just brought my MacBook Air back to life after I managed to torch it with this same issue.

    Thanks so much for your help!

  • Rob

    When I use the delete command, I receive the error message: Ownership of the affected disk is required.
    Nothing is deleted…..

    Any ideas?

  • Rob

    After trying 3 times it worked :)

  • http://twitter.com/hugocbp Hugo Carlos

    I just fixed my MBP because of this post. Thank you a lot!

  • Michael Grundler

    Exactly what I needed. Thank you!