I ran across an issue yesterday in trying to support a Macbook Pro at work. We wanted to reformat the Macbook to a clean system install, but we had no system disk for the computer (it was lost in the move to our new offices), and we didn’t know the admin password for the Mac. I thought we were out of luck, until I ran across a solution in Apple’s discussion forums that showed me how to reset the admin password without a system disk. I figured I’d share this process here, hoping it helps some of you out.
First up, you need to reboot your computer while holding the Command and the R keys down. Hold your Mac’s power button down for about six seconds, and it will turn off. Wait a couple of seconds more, and then hold down Command-R on your keyboard while pressing the power button again to turn the machine on. Keep holding the Command-R combination until you see the little spinning circle just below the Apple logo on the boot up screen.
Once your Mac boots, you’ll see a window with various options. Use your mouse to select Terminal from the Utilities menu, and Terminal will launch. Type in
and you’ll get a dialog box that will let you do just that. Choose the user you want to change the password for, and type in the new password twice. Hit OK, close the reset password window, and reboot your Mac.
A couple things to note here. This has always been possible with a system disk in OS X, so be aware that if someone has physical access to your Mac, they are able to do this. It doesn’t, however, work with FileVault enabled. If you have FileVault set up and forget your password, your stuff is gone for good.
Once I was able to reset the password, I was able to format the hard drive with the Disk Utility, then reinstall OS X Lion (which is what came with the Mac, and then finally install OS X Mountain Lion, which we had purchased for the Macbook via the Mac App Store.
Source: Apple Discussions.