Padlock prevents Disk Recovery access.

Was the computer set up for sale using the What to do before selling or giving away your Mac process. If it was not then you need to do that before you do anything. When buying a used Mac ONLY purchase from a reputable reseller such as Macsales.com or even better from Apple directly by using their refurbished Mac section.

I would suggest one of two things.

1. Return the computer and get your money back and then buy from a reputable source, if you want to buy used there are multiple options available.
2. Take the computer to your local Apple Authorized Service Provider to have the computer professionally diagnosed and fixed.

Because a computer that is not properly setup for sale or trade-in leave you with a mess that is extremely difficult to clean up, it leaves parts of his AppleID attached to it, it is not logged out of important accounts and basically create a huge mess for the next owner, in this case you.

It sounds like you are not 100% sure the previous owner did what was outlined in Apple advice letter I referred to. That means you need to do it which is what I would recommend you do before you go any further, at least then you know for sure you are starting off with a fresh system.

Best of luck.

Australopithicus01 wrote:

Seeing the iMac stuck at partial installation he asked me to re-start disk recovery. I did so and there was the padlock. “Now enter your password, no not your Administrator password or your Apple Account password; your EFI Firmware password”. Happily, I remembered it and did so, and there we were in familiar territory of disk recovery.

Since you are not the original owner of this Mac, make sure never to forget the firmware password or you will never be able to remove it. Apple won't remove it for you since you will not have the necessary proof of purchase to prove to Apple that you are the legal owner of this Mac.

Keep in mind anyone with access to your Mac can turn on the firmware lock. In some ways having it locked with a firmware password you know will prevent the Mac from being locked by a password you don't know. Although I believe macOS 11.x+ does make users authenticate before booting into Recovery Mode so only people that know an admin password for any admin user accounts on your Mac would be able to even boot into Recovery Mode to enable a firmware lock if I'm remembering this correctly (less risky than it was with older versions of macOS depending on who has admin access for your Mac).

If you bought your used Mac directly from Apple, then you should have valid proof of purchase which should make all this a non-issue.

Thanks again HWTech. I have not read about those details. As a second owner and a long time Mac user (30 years), the ease with which one managed the OS security has increased from incremental in early iterations of OSX to complex and more thorough with the implementation of T2 in Monterey. And with double layer sign-in the whole Mac experience is much like Google.

I long for a simpler life but I will not get it from digital technology where ongoing security battles make life so complicated.

My copy of OS11 allows me to access DiskRecovery without a log-in. Of course I have to log into my Mac account before accessing Disk Recovery. I note Big Sur does not allow Automatic Log-in which may be what you are referring to in your observations.

Thank you for your helpful comments.

Cheers,