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I have a 1st-gen MacBook Air. It came with, if I remember correctly, Leopard. I subsequently upgraded to Snow Leopard and then to Lion.

However, I have found it to be very sluggish after upgrading to Lion, so I decided to downgrade it to Snow Leopard.

Since doing so, I can no longer log in.

Here's what I did

  1. Use SuperDuper! to make a bootable backup of my MacBook Air hard drive on a partition on an external USB hard drive
  2. Boot using the aforementioned partition on external USB hard drive as startup disk to verify the clone. Verified.
  3. On another Mac (one with a DVD drive), use SuperDuper! to clone the Mac OS X Snow Leopard install DVD onto another partition onto the same external USB hard drive.
  4. On the MacBook Air, boot into the Install partition on the external drive.
  5. Use Disk Utility to re-partition the MacBook Air's internal hard drive.
  6. Install Mac OS X Snow Leopard on the MacBook Air's hard drive
  7. When asked as part of the installation, choose to migrate users, applications and data from the other partition on the same external hard drive that contained the previously backed up hard drive
  8. Restart the MacBook Air on successful installation
  9. Attempt to log in with the password I originally had.

The login screen simply shakes its head at me and does not let me log in.

With my limited knowledge, I imagine that something has gone awry with password hashing. Any ideas what I can do?

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Update: I booted into single-user mode (cmd-s at startup) and changed my password with 'passwd'. It still did not work –  Vihung Aug 14 '12 at 18:14
    
Update 2: Today, I tried to boot the laptop to follow the suggestion given by Naishy. However, while booting into single-user mode (cmd-s at startup) I now get unending messages of the form "Waiting for Root Device". Force-shutting it down and rebooting it normally resulted in the usual grey screen with the apple logo, followed by a grey screen with a crossed circle. Booting from the external drive and running repair- and verify- disk did not help fix this. –  Vihung Aug 17 '12 at 11:04
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1 Answer

You may want to try the method described here under 'Reset Mac Password – without a CD'. This method resets the OS so that it thinks that it's the first time it's been run. This will kick off the Create New User Account wizard when the OS is rebooted, which allows you to create a new account with Administrator rights. You can then use this account to reset the password of the original account.

Extract from above article:

Stage 1) Boot into Single User Mode and remove a setup file

  • Restart the Mac holding down the Command+S keys, this will take you into Single User Mode and it’s Terminal interface
  • You’ll need to check the filesystem first: fsck -fy

  • Next, you must mount the root drive as writeable so that changes will save: mount -uw /

  • Now, type the following command exactly, followed by the enter key: rm /var/db/.applesetupdone

  • After removing the applesetupdone file, you need to reboot, type ‘reboot’ and hit enter.

Stage 2) Create a New User Account upon System Boot

You aren’t finished, but the hard part is now over – no more command lines, you’ll now be in the familiar Mac OS X GUI to finish the password reset process. In this step we just create a new user account as if you just got a new Mac:

  • Upon reboot, you will be presented with the traditional “Welcome Wizard” startup screen just like when you first get a Mac
  • Follow the welcome wizard and create a new user account – making the account name different from the account whose password you want to recover
  • Continue on and boot into Mac OS X with this newly created user account, this new user account is an Administrator and has administrative access

Stage 3) Reset the Forgot Password via System Preferences

You are almost done, now you just need to reset the forgotten user account password using the Accounts control panel:

  • Once you are booted into Mac OS X, click on the Apple logo and then navigate down to “System Preferences”
  • Click on the “Accounts” icon in System Preferences
  • Click on the Lock icon in the lower left corner of the “Accounts” preference window and enter the newly created user credentials, this enables you to change other user accounts and reset other users passwords
  • On the left side user panel, select the user account containing the forgotten password
  • With the user of the forgotten password account selected, click on the “Reset Password” button
  • Enter a new password for that user, be sure to include a meaningful hint so you don’t forget it again!
  • Close System Preferences and reboot the Mac
  • You can now login to the previously inaccessible user account using the newly reset password! All user files and settings are maintained as before the password was forgotten
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I tried the method stated on Apple's knowledgebase with a similar title to your suggestion - Update: I booted into single-user mode (cmd-s at startup) and changed my password with 'passwd'. It still did not work - I will try your suggested method and see how things go. The nuance here is to remove the .applesetupdone file and to create a new user in the GUI –  Vihung Aug 15 '12 at 15:04
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