MacOS – How to delete a new Admin account and its data without knowing the old admin password

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I used my friend's Mac by creating a new admin account. Now I need to return it and want to delete the new Admin account I created without knowing his password. In other words, can I delete my own Admin account?

Best Answer

First, create a new Admin user account as your account allows to make it.
Login with the new admin account.
Delete your original account and select the "Delete home folder" option.

Easy way to delete your account with settings, not so easy to delete the new second account but your friend can do that with their admin account (hopefully)

Related Solutions

MacOS – How to get admin access to a Mac without knowing the current password

With macOS Catalina and many recent macOS - the built in tools guide everyone how to reset admin permissions without needing any of the single user mode boot tricks of old (left at the bottom for posterity and people running OS that were shipped before 2015)

Now - follow the official Apple triage steps: https://support.apple.com/en-us/HT202860

Instead of single user mode, boot to recovery and use the Terminal Utility. Then you can remove the .AppleSetupDone file from:

/Volumes/Macintosh\ HD\ -\ Data/private/var/db

There are many ways to reset the login password without installation media, here are three:

Change the password in single user mode

Hold ⌘+S on startup
mount -uw / (fsck -fy is not needed)
launchctl load /System/Library/LaunchDaemons/com.apple.opendirectoryd.plist (or /System/Library/LaunchDaemons/com.apple.DirectoryServices.plist in 10.6)
dscl . passwd /Users/username (without a trailing slash) and enter a new password. You can ignore the error about com.apple.DirectoryServices.plist.
reboot

Create a new admin account

Hold ⌘+S on startup
mount -uw / (fsck -fy is not needed)
rm /var/db/.AppleSetupDone
reboot
Go through the steps of creating a new account. You can leave all fields in the personal information step blank.
After logging in to the new account, go to the Users & Groups preference pane
Select the old account, press the Reset Password... button, and enter a new password

Restart and enter Recovery mode (for 10.7 Lion and newer OS only)

Hold ⌘+R on startup
Open Terminal from the Utilities menu
Type resetpassword and follow the instructions

Using iCloud Recovery Mode (10.9.x & newer)

After incorrectly entering a user password three times at the Mac login screen, a message appears saying "If you forgot your password, you can reset it using your Apple ID"
Click the arrow button to bring up a "Reset Password" dialog
Enter the Apple ID login information that is tied to the OS X user account and click “Reset Password”
Click “OK” to confirm the new keychain creation
Enter and verify a new password, fill out the password hint field, and again click on “Reset Password”
Click “Continue Log In” to login as the user account with the reset password

New MacBook had user accounts already; I deleted admin and can’t get back in

I would suggest you to reinstall OS X and doing it not on the top of the existing version. Start Mac in Recovery mode (hold Cmd-R keys pressed on boot). When in Recovery mode - select Disk Utility, format your disk, exit Disk Utility and select Install or reinstall OS X.

P.S. If you have your files stored in Mac - backup ones before formatting the disk.