Disk Utility won’t let you erase or repartition until you unlock or decrypt.
1. Launch Terminal.app and type the following command:
diskutil cs list
This will give you with a list of the CoreStorage volumes on your system. Unless you have a Fusion drive or multiple encrypted drives, your FileVault 2-encrypted drive should be the only one listed.
In the listing, you will want to select and copy the Logical Volume Group (LVG) alphanumeric UUID for your CoreStorage volume. The LVGshould be the first UUID listed and it’s the one we want to delete.
2. In Terminal.app type the following command:
diskutil cs delete UUID_here
This will delete your CoreStorage volume and reformat it as an unencrypted HFS+ volume.