Starting in OS X El Capitan, Apple overhauled Disk Utility’s various functions to add new features and remove others. As of macOS Sierra, it appeared at first that the abilities to unlock or decrypt a FileVault 2-encrypted drive had both been removed from Disk Utility. After some investigation though, it looks like the ability to decrypt has been removed, but you can still unlock using Sierra’s Disk Utility. For more details, see below the jump.
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 and use any other 10.12.x-booting drive. As long as you have macOS Sierra’s Disk Utility, the process below should work.
2. Open Disk Utility.
3. Select your locked FileVault 2-encrypted boot drive.
4. Under the File menu, select Mount to mount the drive. The mount attempt should generate a password prompt to unlock the encrypted drive.
5. When prompted for a password, you can enter the password of any FileVault 2-enabled account on the drive.
6. Once you have unlocked the drive, you should then be able to use Disk Utility’s repair tools to hopefully fix whatever problem your Mac is having.
