Using Disk Utility to unlock or decrypt your FileVault 2-encrypted boot drive
Bad things happen and sometimes those bad things cause your FileVault 2-encrypted Mac to be unbootable. In the event that you find yourself in this place, or you’re about to be, here’s how you can unlock or decrypt your FileVault 2-encrypted drive using Disk Utility and the password of an account that’s authorized to log in at the FileVault 2 pre-boot login screen.
Update – June 11, 2013: As of Mac OS X 10.8.4, you will need to unlock the encrypted volume first, then you will be able to decrypt it. See this post for details.
1. Boot your Mac and hold down ⌘-R (Command –R) to boot from the Mac’s Recovery HD partition.
Note: You can also boot from a 10.7 installer drive , boot to Target Disk Mode and connect it via Firewire or Thunderbolt to another Mac, or use some other 10.7-booting drive. As long as you have 10.7′s Disk Utility, this should work.
2. Open Disk Utility.
3. Select your locked hard drive.
4. Under the File menu, select Unlock “Drive Name” or Turn Off Encryption… , based on what you want to do.
5. When prompted for a password, you can enter the password of any authorized account on the drive.
6. If you unlock the drive, you should then be able to use Disk Utility’s repair tools to hopefully fix the problem that’s preventing your Mac from booting.
7. If you turn off the encryption, the encrypted drive will decrypt. Once it’s finished decrypting, you should be able to access your data again using normal recovery methods (booting from another 10.6 or 10.7 boot drive, utility drive, etc.)