How to cope with a FileVault recovery key disappearing while you write it down
FileVault is an extraordinary bit of macOS technology. Introduced years ago, it encrypts the entire contents of your startup volume so that when the data is at rest—when your Mac is powered down—the drive is effectively full of garbage nonsense to anyone who doesn’t possess either the password to an account authorized to log in via FileVault or the special recovery key set when you turn FileVault on.
When you use the Security & Privacy preference pane’s FileVault tab to enable this encryption, macOS prompts you with two choices:
Allow my iCloud account to unlock my disk