All users disappear after install

I too lost all user accounts including the admin.

None of my passwords work even for guest!

I can even change guest password using the admin password because it won't recognize it.

I HAVE A BRICK!!!!!!!!!!!!

HELP APPLE!!!!!!!

This is a User to User forum and no one from Apple will be able to help you here. Try contacting AppleCare at expresslane.apple.com

Same issue for me - not happy - wish my mini had already had enough RAM to use as the test - I was nt using Filevault - do have a time machine backup - and wsa considering a clean install - but this is a real problem and if there is not an easy fix - this could be a HUGE disaster for LOTS of people. Could be remembered as the OS X upgrade that Apple botched.

seems to be working for me - not 100% done with the process - but positive progress so far.

Thanks for the pointer! The advice at MacRumours worked for me, and I'm back in (phew!)

My computer will not even go into recovery mode. Keeps going right to the login screen.

My install reverted to admin/user accounts I thought had been erased a year ago. I could log in but I'd lost Libraries, Keychain and Documents. Now restored 10.6.8. I'll try the macrumours solution. Very unhappy.

Here's a slightly cleaned up version of the instructions from the MacRumors link above:

• Reboot your mac, hold down Command R just as it chimes (or just as it starts up if you have volume off)

• Select "Terminal" from the Utilities menu

• Type resetpassword and hit Return

• Select root and enter a root password

• Go to Apple Menu > Restart

• Log in as root using the password you just created

• Go in System Preferences > Users & Groups

• Create a new Administrator account using the same exact short name and password as your old user account (same capitalization of short name is important)

• When you accept it will say "The Users folder already contains a folder with the same name as the home folder for this account"

• Select "Use Existing Folder" (and wait for a while if you have a lot of files

• Log out or reboot and you should see your old user there, try logging in

Confirmed - this works. Thanks.

When I try to create a new ticket on expresslane.apple.com it asks for serial number - any idea how can I get serial number without logging in?

This issue is unbelievable! Apple should refund the cost of the upgrade to all of us! I am backing up all my data before trying the recovery procedure described above... you never know.

The serial number of the computer? Depends on the model; on my Air it's printed on the bottom. On my iMac I think it was on the cardboard box it came in. Try here: http://support.apple.com/kb/ht1349 for more help.