Password reset loop

Please read this whole message before doing anything.

If the machine is a MacBook and you get a black screen without a cursor, you might be able to log in by closing the lid for about a minute, then opening it.

The easiest way by far to recover is to restore the startup volume from a backup that predates the issue. If you can't do that or don't want to, the following procedure works sometimes. There's no assurance that it will work in your case. It's only suitable for advanced users. I can't offer instructions any more detailed than those below.

Back up all data if you haven’t already done so. There are ways to back up, even if you can’t log in. Ask for guidance if you need it. Do not attempt this procedure unless you have a current backup of all data.

Briefly, you need to rename the following folder:

/Library/Preferences

If you already know how to do that, you can skip the rest of this message. Do it, and try again to log in.

Step 1

Start up in single-user mode by holding down the key combination command-S at the startup chime. If you’re using an external USB keyboard, it must be plugged directly into a built-in port on the Mac, not into a hub. Release the keys when you see a black screen with scrolling white text. In this mode, there’s no graphical interface; just a command line. Initially, you can’t make any changes to files.

Step 2

When the text stops scrolling, and you see a line ending in a pound sign (“#”), type the following command:

mount -uw /

You must type the command exactly as given, with no mistakes.

Press the return key.

Step 3

Enter the following command:

mv /L*/Preferences /Library/Preferences.old

Again, you must get it exactly right.

Step 4

Enter the command:

exit

Press return. The text will start scrolling again, and then the system will restart. Try again to log in. If successful, you can experiment with restoring some of the contents of the Preferences folder from a backup, or if necessary, from the renamed folder Preferences.old, although you may have trouble with permissions if you do the latter. Otherwise, just recreate all system-wide settings and delete Preferences.old by dragging it to the Trash.

If you want to preserve the data on the startup drive, and it's not already backed up, you must try to back up now, before you do anything else. It may or may not be possible. If you don't care about the data, you can skip this step.

There are several ways to back up a Mac that is not fully functional. You need an external hard drive or other storage device to hold the data.

1. Start up from the Recovery partition, from Internet Recovery, or from a local Time Machine backup volume (option key at startup.) Launch Disk Utility and follow the instructions in this support article, under “Instructions for backing up to an external hard disk via Disk Utility.” The article refers to starting up from a DVD, but the procedure in Recovery mode is the same. You don't need a DVD if you're running OS X 10.7 or later.

If you use FileVault 2, then you must first unlock the startup volume. Select its icon ("Macintosh HD," unless you gave it a different name.) It will be nested below another disk icon, usually with the same name. Click the Unlock button in the toolbar. Enter your login password when prompted.

2. If Method 1 fails because of disk errors, then you may be able to salvage some of your files by copying them in the Finder. If you already have an external drive with OS X installed, start up from it. Otherwise, if you have Internet access, follow the instructions on this page to prepare the external drive and install OS X on it. You'll use the Recovery installer, rather than downloading it from the App Store.

3. If you have access to a working Mac, and both it and the non-working Mac have FireWire or Thunderbolt ports, start the non-working Mac in target disk mode. Use the working Mac to copy the data to another drive. This technique won't work with USB, Ethernet, Wi-Fi, or Bluetooth.

4. If the internal drive of the non-working Mac is user-replaceable, remove it and mount it in an external enclosure or drive dock. Use another Mac to copy the data.