Aqui está algo que você pode experimentar no artigo do hackmac.org " Acesso ao modo de usuário único quando bloqueado ":
Procedure
1) Change the Amount of RAM
Change the amount of RAM inside the computer. It doesn’t matter if you add RAM or remove RAM, but the amount of RAM has to be different than the amount present when the computer was last turned off.
Be careful when handling RAM. It is very sensitive to static electricity.
2) Reset the PRAM and NVRAM
Once you have successfully changed the amount of RAM, and preferably closed the case back up, turn the computer on. Once you hear the chime signaling that the computer is turning on hold CMD+Option+P+R. Hold them down until the computer restarts and you hear the chime for a second time. Immediately after you hear this second chime release the keys, then quickly press them again. Wait until it restarts and you hear a third chime. Then repeat for a fourth time. (Note: This is resetting your computer’s PRAM and NVRAM)
3) Boot into Single-User Mode
Once you have heard the fourth chime hold CMD+S. This will boot you into single-user mode. From here you can perform whatever actions you want to do, but be careful not to change anything vital to the running of the computer.
4) Replace/Remove RAM
Turn the computer off and put the original amount of RAM back inside. This step isn’t necessary but is highly recommended.You should try to leave the computer as close to its original state as possible.
Again remember that opening up a Mac does void its warranty.