So first, figure out if you have automatic login on. If you do, you’ll never see your login screen at all, so go to System Preferences > Users & Groups to check. Choose Login Options from there, and if “Automatic login” is on, click on the padlock in the lower-left corner of the window to unlock the pane, then toggle the option off. You can now follow along with this tip, and there’s a bonus—your data is now a bit safer from thieves if your Mac gets stolen. This’ll mean that every time you restart your Mac, you’ll need to enter your password, but that small layer of security is worth it, right?
After that, the first neat thing you can do is in System Preferences > Security & Privacy under the “General” tab, where you’ll need to unlock things with the padlock as before if you want to make changes. When you then turn “Show a message when the screen is locked” on and type something in the box below that, your message will show up on the login window. It’ll also appear if your Mac requires a password to unlock the screen saver or wake it from sleep.
Change this…
…for your note to show up here.
As you can see, my message gives information about how to contact me if my machine is found. You, of course, can choose something different—a warning about how Find My Mac works, perhaps, or some other vague threat. Whatever floats your proverbial boat.
The other cool thing you can do in the same vein is to add an agreement that a user would have to accept before logging in. Obviously this would be most useful if you were managing a large number of machines, but heck, making your family members laugh is just as important.
Here’s how you go about it. First, create a file in one of three formats: .rtfd, .rtf, or .txt (TextEdit is great for this). Add whatever content you want, and then save the file as “PolicyBanner” in the Library > Security folder.
You’ll have to type in your password before Mac OS X will let you put anything in there, but after you do that, your task is finished. Of course, you’ll want to log out or restart to see your handiwork.
To undo your changes, just delete the file you put in your Library > Security folder, and things will go back to normal. And now your 2012 will be filled with mirth as your spouse has to accept your crazy agreements before logging on. Or, conversely, your 2012 could be filled with horror and arguments if you step over the line from “funny” to “offensive.” That’s a line I may know a thing or two about.