If you are locked out of an account which uses FileVault due to forgetting your password and the master password then this will not help you.
This only applies to currently accessible accounts and accounts created afterwards.
To remove the current Master Password:
- Login as an administrator account.
- Click the Finder icon on the Dock.
- On the top menu, go to Go -> Go to Folder…
- Type in /Library/Keychains/ and press Enter.
- Delete the FileVaultMaster.keychain and FileVaultMaster.cer files.
To enter a new Master Password:
- Follow the instructions above if you haven’t already.
- Click the Apple icon in the top left of the screen.
- Click System Preferences.
- Click on Security.
- Click the FileVault tab.
- Click the “Set Master Password…” button.
- Follow the instructions on-screen.
To apply the new Master Password to an account which has FileVault enabled:
- Click the Apple icon in the top left of the screen.
- Click System Preferences.
- Click on Security.
- Click the FileVault tab.
- Click “Turn Off FileVault…”
- Follow the prompts and wait for the decryption process to finish.
- You will be taken to the login window. Log back in.
- Click “Turn On FileVault…”
- Follow the prompts and wait for the encryption process to finish.
- You will be taken back to the login window again. Log back in.