Your 27-inch iMac includes the Apple T2 Security Chip, which provides the following security features to protect what’s on your computer and prevent unauthorized software apps from loading during startup:
Secure storage: The storage drive of your 27-inch iMac is encrypted with hardware keys to provide advanced levels of security. In the event of a catastrophic failure, data recovery may not be possible, so you need to back up your files to an external source. See the Apple Support article About Secure Boot.
If your 27-inch iMac doesn’t start up because it detects an untrusted component, it will start up from a secure recovery partition and automatically correct issues if possible. To learn more about the Startup Security Utility or to find out how to set other options, such as starting up from an external device, see the Apple Support article About the Apple T2 Security Chip.