Using Disk Utility on macOS Sierra to unlock FileVault 2-encrypted boot drives
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.
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Der Flounder 
Seems Apple have created further issues with Sierra.
Cant enter password info for step 5 as keyboard doesnt work, also usb keyboard no longer works. How do Apple think a password can be entered? Shouldn’t be this much drama, just to run a disk fix program surely?
Must i have to remove the drive and scan it from another mac? Backwards if you ask me.
Not sure what else to try though. . . Well done Apple!
Ok, after further attempts, you must boot into recover mode without the usb keyboard attached, enter Disk utility then attach the usb keyboard and it will work.
Must be whatever stops the main keyboard working also affecting the usb one if connected from the start.
Hurrah!
My MB pro is crashing at step 5 immediately on entering the password to unlock the volume.
Running diskutil apfs unlockVolume disk2s1 prompts me for a password but on entering password the MacBook restarts. If I attempt a wrong password it prompts me for correct password. On entering correct password it crashes my MacBook .
Any ideas what to do?
Not work at all
Thank you for this post. Exactly what I needed to fix my Mac Book 2009 A1342. My ‘Macintosh HD’ was showing up grayed out in the reboot mode but yet that is what disc utility wanted me to fix. Your instructions on how to ‘mount’ worked exactly as given. Many thanks!