Mac OS X : Password Does Not Work As Expected After a Change

When using Mac OS X Server, the Setup Assistant creates both Admin and root user accounts with the same password. For Mac OS X (client), the root password remains undefined until you enable the root user. In both versions of Mac OS X, subsequent changes to a password only affect the selected user, not both.
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Symptom

After changing your password, the previous password continues to work for certain functions.


Solution

During the installation process, the Mac OS X or Mac OS X Server Setup Assistant asks for an Administrator account name and password. The Setup Assistant then creates both a "root" user and an "Admin" user. For Mac OS X Server, the Admin and root accounts created both have the same password and root is enabled by default. For Mac OS X (client), the root password remains undefined until you enable root, which is disabled by default. Only the Admin user is immediately visible to you in either version of Mac OS X.

When you change the password at a later time, only the password of the selected user is changed. The password of the other user remains in effect until you change it.

The two users have different capabilities, login names, and visibility characteristics.
Admin User
root User
DescriptionThe "Admin" user is created for day-to-day login and operation of your computer. The Admin user can log in to the Server Admin and Macintosh Manager applications, but the root user cannot. You can create multiple Admin users in the Users pane of System Preferences.The "root" user can perform certain administrative tasks that are restricted from the Admin user, such as installing WebObjects software or setting up the NetInfo database. The root user is enabled by default in Mac OS X Server but not in Mac OS X (client). You can have only one root user.
Login NameThe Admin user takes the name you set in the Setup Assistant, which may be changed in the Users control panel.Type the name "root" or "System Administrator" to log in as the root user.
Visibility The Admin user name appears in the Users pane of System Preferences regardless of who is logged in to the computer. Admin users have Home directories that appear in the Finder.The root user only appears in the Users pane of System Preferences (as "System Administrator") when you are logged in to the computer as root. The root Home directory (/Private/var/root/) only appears in the Finder when logged in to the computer as root.
Password ChangeFollow these steps:
    1. Choose System Preferences from the Apple menu.
    2. Click Users.
    3. If necessary, click the padlock to make changes.
    4. Select the user designated Admin.
    5. Click Edit User.
    6. Type a new password in the Password and Verify fields.
    7. Click OK.
Follow these steps:
    1. Choose Log Out from the Apple menu.
    2. Log in as the root user.
    3. Choose System Preferences from the Apple menu.
    4. Click Users.
    5. Select the user named System Administrator.
    6. Click Edit User.
    7. Type a new password in the Password and Verify fields.
    8. Click OK.

Important
    1. Do not confuse "Admin" with "System Administrator," which is actually an alternative name for the root user (see table above).

    2. For more information on the root user, including precautionary statements, please see:

Article 106290: "Mac OS X: About the root User and How to Enable It"
Published Date: Feb 17, 2012