[Perdi acesso administrador -- Mudei o nome do usuário]

Esse tutorial serviu pra mim.

 

1. Shut down your Mac
2. Power up your Mac and immediately press and hold the command + S keys
3. Keep the keys down until you see text on the screen

4. Now enter the following command:

   /sbin/fsck -fy

   and press enter

5. Next enter the following command:

   /sbin/mount -uw /

   and press enter

6. Now enter the following command:

   rm /var/db/.AppleSetupDone

   and press enter

7. Finally, enter the reboot command

   and press enter

Your Mac should reboot and you can log in normally, after which point the Setup Assistant will launch. Now you need to create a new admin user. Don't worry, your other user accounts should be intact. NOTE: You should choose a new user name for this admin account.

Once you're back up and running you can go to Apple > System Preferences > Users & Groups and select the Allow user to administer this computer checkbox for the 'grant' user.

[Perdi acesso administrador -- Mudei o nome do usuário]

Aconteceu exatamente a mesma coisa comigo... estou buscando a solução, caso descubra algo por favor poste aqui