One of my backup disks is full. How can I Erase & Reformat the drive? I use Yosemite OS.

The External Hard Drive was formatted as Partitioned Mac OS Extended (case sensitive, Journaled, Encrypted volume) Disk.

I tried under "Disk Utilities", but got this Error Message:

Disk Encryption failed with the error: Unable to delete the Core Storage logical volume group.

  • As first step you should turn off the encryption. Did you do that ?
    – Ruskes
    Commented May 8, 2015 at 4:55
  • No.Do you mean Turn Off the Mac Encryption in Fire Vault?
    – DaR
    Commented May 8, 2015 at 8:58
  • No.Do you mean Turn Off the Mac Encryption in Fire Vault?Or, Backup Disk Encryption via Time Machine?
    – DaR
    Commented May 8, 2015 at 8:59
  • You could enter the gpt command in a Terminal window to erase all partitions from the external disk. This would require you to be comfortable using Terminal. Commented May 8, 2015 at 9:05
  • DaR, if you what Buscar웃 to be notified that you posted a comment, you need include @Buscar somewhere in your comment. Commented May 8, 2015 at 9:15

1 Answer 1


This is an example of a procedure to erase an external disk containing a single encrypted partition.

  1. Open the Disk Utility application and highlight the "Logical Volume Group" representing your external disk. An example is shown below. (Hint: click on image for a better view.)

  2. Next, click the Info icon to get the pop up window. Note, in the example shown below, the "Physical Backing" is disk1s2. This means the physical disk number is 1. The "s2" suffix can be ignored. Your numbers may be different.

  3. Next, highlight the "Encrypted Logical Partition" as shown in the example below.

  4. Again, click the Info icon to get a pop up window. Note the "Disk Identifier". In the example shown below, the value is disk2. Your value may be different.

  5. Close the Disk Utility and open the Terminal application. Enter the following commands. If your numbers differed from mine, make the appropriate substitutions. If prompted, enter your login password.

    diskutil  unmountDisk  /dev/disk2
    sudo  gpt  destroy  /dev/disk1
    sudo  gpt  create  -f  /dev/disk1

    When I entered these commands, I saw the following in the Terminal window.

    Steelhead:~ davidanderson$ diskutil  unmountDisk  /dev/disk2
    Unmount of all volumes on disk2 was successful
    Steelhead:~ davidanderson$ sudo  gpt  destroy  /dev/disk1
    Steelhead:~ davidanderson$ sudo  gpt  create  -f  /dev/disk1
    Steelhead:~ davidanderson$ 

    The distutil command must successfully unmount the disk in order for the gpt commands to work. If the disk will not unmount, then some window needs to be closed. Do not proceed with the gpt commands, if you can not unmount the disk.

  6. If successful, you should see a result similar to what is shown below. The result shown is a 4GB flash drive with no partitions. You can either add a partition or repartition the entire drive.

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