welcome

- Hope this helps out.

This is going to sound ridiculous but I was testing on my first Mac and wanted to play around with this hidden user account feature. After following the notes on this thread and lowering the UID to below 500, I ran the terminal command stated above to add it to the Hidden list. Now, for the life of me, I can't figure out how to reverse the process so I can access the account again under Sys Preferences.

Any help would be greatly appreciated. I'm pulling my hair out in frustration.

Thanks!

You'll need to re-image more than likely. once the UID has been lowered (and machine restarted), the change is propagated throughout the OS and it can't be restored unless you re-image the HD with the OS again and start fresh. Don't perform this mod unless you're serious about managing your administrator account.

No need to reimage.
To reverse the situation,
sudo defaults delete /Library/Preferences/com.apple.loginwindow Hide500Users

This will make the account visible in the Accounts preference pane unless you had added the HiddenUsersList dealie someone else suggested, in which case:
sudo defaults delete /Library/Preferences/com.apple.loginwindow HiddenUsersList

should do the trick.

Log in as a different admin (create one if necessary?) and control+click the previously hidden user icon in Accounts pref pane and change the user account ID, and/or home folder location.

If you change the uid, you will need to chown the home folder to the new uid by:
sudo chown <adminusername> /path/to/home/folder

If you changed the home folder location, you'll either move the old home folder to the new location, or allow the system to create a new home folder when you log in as the previously hidden admin user (if you created a second admin account that you no longer need, delete the second admin account through Accounts pref pane) and dispose of the old home folder.

I hope that's helpful. It can all be scripted also (though I doubt there would be much call for that).