Error BigSur update

Set a firmware password on your Mac

When you set a firmware password, users who don't have the password can't start up from any disk other than the designated startup disk.

This feature requires a Mac with an Intel processor. For the equivalent level of security on a Mac with Apple silicon, simply turn on FileVault.

How to turn on a firmware password

1. Start up from macOS Recovery.
2. When the utilities window appears, click Utilities in the menu bar, then choose Startup Security Utility or Firmware Password Utility.
3. Click Turn On Firmware Password.
4. Enter a firmware password in the fields provided, then click Set Password. Remember this password.
5. Quit the utility, then choose Apple menu  > Restart.

Your Mac asks for the firmware password only when attempting to start up from a storage device other than the one selected in Startup Disk preferences, or when starting up from macOS Recovery. Enter the firmware password when you see the lock and password field:

How to turn off a firmware password

1. Start up from macOS Recovery. During startup, you will be prompted to enter the firmware password.
2. When the utilities window appears, click Utilities in the menu bar, then choose Startup Security Utility or Firmware Password Utility.
3. Click Turn Off Firmware Password.
4. Enter the firmware password again when prompted.
5. Quit the utility, then choose Apple menu  > Restart.

If you forgot your firmware password

If you can't remember your firmware password, schedule an in-person service appointment with an Apple Store or Apple Authorized Service Provider. Bring your Mac to the appointment, and bring your original receipt or invoice as proof of purchase.

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Writing an effective Apple Support Communities question

Welcome !

So was that the answer to your question ??

Thank you too :-))

Personally, would keep a copy of the Firmware password in my Safety Deposit Box at Bank and another on iPhone using a six digit alpha numeric PIN for the iPhone. The back up, at Bank - would be better even if had to wait for the weekend to pass - better than the Proof of Ownership and trip to Apple Store

Very Best and hope it is resolved soon

Good Question and do not know. Imagine they may explain the process if you ask when there. Would in interested in knowing - it time permits.